The African elephant, the largest land animal remaining on earth , is of great importance to African ecosystem(生态系统). Unlike other animals, the African elephant is to a great extent the builder of its environment. As a big plant-eater, it largely shapes the forest-and-savanna(大草原)surroundings in which it lives, therefore setting the terms of existence for millions of other animals that live in its habitat(栖息地).

    It is the elephant's great desire for food that makes it a disturber of the environment and an important builder of its habitat. In its continuous search for the 300 pounds of plants it must have every day, it kills small trees and underbushes, and pulls branches off big trees. This results in numerous open spaces in both deep tropical forests and in the woodlands that cover part of the African savannas. In these open spaces are numerous plants in various stages of growth that attract a variety of other plant-eaters.

    Take the rain forests for example. In their natural state, the spreading branches overhead shut out sunlight and prevent the growth of plants on the forest floor. By pulling down trees and eating plants, elephants make open spaces, allowing new plants to grow on the forest floor. In such situations, the forests become suitable for large hoofed plant-eaters to move around and for small plant-eaters to get their food as well.

    What worries scientists now is that the African elephant has become an endangered species. If the elephant disappears, scientists say, many other animals will also disappear from vast areas of forest and savanna, greatly changing and worsening the whole ecosystem.

    In 1905, as part of his Special Theory of Relativity, Albert Einstein published the point that a large amount of energy could be released from a small amount of matter. This was expressed by equation E = me 2(energy = mass times the speed of light squared). But bombs were not what Einstein had in mind when he published this equation.

    In 1929, he publicly declared that if a war broke out he would “refused to do war service, direct or indirect…” His position would change in 1933, as the result of Adolf Hitler's coming into power in Germany.

    Einstein's greatest role in the invention of the atomic bomb was signing a letter to President Franklin Roosevelt urging that the bomb be built because some physicists feared that Germany might be working on an atomic bomb. Among those concerned were physicists Leo Szilard and Eugene Wigner. But Szilard and Wigner had no influence with those in power. So in July 1939 they explained the problem to someone who did: Albert Einstein. After talking with Einstein, in August 1939 Szilard wrote a letter to President Roosevelt with Einstein's signature on it, which was delivered to Roosevelt in October 1939.

    Germany has invaded Poland the previous month; the time was ripe for action. That October research of a-bomb began but proceeded slowly because the invention of the atomic bomb seemed distant and unlikely. In April 1940 an Einstein letter, ghost-written by Szilard, pressed the researchers on the need for “greater speed”.

    As the realization of nuclear weapons grew near, Einstein looked beyond the current war to future problems that such weapons could bring. He wrote to his friend about his fear about the future use of the a-bomb.

    The atomic bombings of Japan occurred three months after Germany gave in.

In November 1954, five months before his death, Einstein summarized his feelings about his role in the creation of the atomic bomb: “I made one great mistake in my life… when I signed a letter to President Roosevelt recommending that atom bombs be made; but there was some justification—- the danger that the Germans would make them”.

根据短文内容，选择最佳答案，并将选定答案的字母标号填在题前括号内。

    It's true that quite a few most respected scientific authorities have confirmed that the world is becoming hotter and hotter. There^'s also strong evidence that humans are contributing to the warming. Countless recent reports have proved the same thing. For instance, a 2010 summary about the climate science by the Royal Society noted that: The global warming over the last half - century has been caused mainly by human activity.”

    You may not believe that humans could change the planet's climate, but the basic science is well understood. Each year, billions of tons of greenhouse gases are released into the atmosphere because of human activity. As has been known for years, these gases hold heat that would otherwise escape to space, wrapping the planet in an invisible blanket.

    Of course, the earth's climate has always been changing due to “natural” factors such as volcanic eruption or changes in solar, or cycles concerning the Earth's going around the sun. According to the scientific research, however, the warming observed by now matches the pattern of warming we would expect from a build - up of greenhouse gas in the atmosphere - not the warming we would expect from other possible causes.

    Even if scientists did discover another reasonable explanation for the warming recorded so far, that would give birth to a difficult question. As Robert Henson puts it: “If some newly discovered factor is to blame for the climate change, then why aren't carbon dioxide and the other greenhouse gases producing the warming that basic physics tells us they should be?”

    The only way to prove with 100% certainty that humans are responsible for global warming would be to run an experiment with two identical Earths-one with human influence and one without. That obviously isn't possible, and so most scientists are careful not to state human influence as an absolute certainty.

    The United Arab Emirates (UAE) is mostly a desert. However, this has not stopped the country from creating great building projects. On September 26, UAE officials announced another ambitious project — Mars Science City. Expansive deserts and miles of coastline provide plenty of options for safe rocket launches (发射). And its position on the Earth makes it especially appealing as the spin of the Earth provides an extra push, meaning less fuel is needed to get payloads into orbit. All these seem to make it possible.

    The Mars City Project, designed by Bjarke Ingels Group, or BIG, will provide a realistic model to simulate (模仿) living on the surface of Mars, the red planet. It is part of the UAE's Mars 2117 Project to lead the global race to land humans on Mars and be the first to build a settlement there. Sheikh Mohammed bin Rashid, Prime Minister and ruler of Dubai, says, “The UAE seeks international support to develop technologies that benefit humans, and lay the foundation of a better future for more generations to come.”

    Built outside Dubai, the 1.9 million square feet city is expected to cost $135 million. It will consist of several dome-shaped laboratories, similar to the ones imagined for the first Mars settlers. Scientists from around the word will be invited to conduct research to come up with methods to create food, water and energy, using techniques which can be copied onto the red planet. The living spaces, where the researchers can live for up to a year, will simulate the planet's conditions as much as possible.

    The research city will also include a museum featuring famous space achievements to help educate and inspire children to undertake space exploration and discovery. To discover whether the construction method works on Mars, the museum's walls will be 3D printed using the sand from the nearby desert. Now, if they would only add some living quarters for the general public to experience life on Mars without leaving Earth, life would be perfect.

    Scientists have recreated a 1985 study of birds in Peru that shows climate change is pushing them from their natural environment. Thirty years ago, researchers studied more than 400 kinds of birds living on a mountainside in Peru. In 2017, researchers looked again at the bird populations. They found that almost all had moved to higher places in the mountain. Almost all had decreased in size. And, the scientists say at least eight bird groups that move to the higher altitude had died out completely.

    Mark Urban, director of the Center of Biological Risk at the University of Connecticut, said this recent study was the first to prove that rising temperatures and moving to avoid them can lead to extinction.

    In 1985, Fitzpatrick, director of the Cornell Laboratory of Ornithology, and a team of scientists established a camp alongside a river running down a mountainside in southeastern Peru. He wanted to document where tropical bird groups lived. His team spent several weeks using nets to catch and release birds. They kept detailed notes of birds they caught, saw or heard. In 2016, Fitzpatrick passed his notes, photos and other records to Benjamin Freeman. Freeman who has been researching tropical birds for more than 10 years set out in August and September of 2017 to copy Fitzpatrick's study. His team used the same methods, searching the same places in the same time of year.

    Freeman said that the birds moved an average of 98 meters further up the mountain, believing that temperature is the main cause of the birds' movement. Fitzpatrick noted that birds used to living in areas with little temperature change might be especially at risk because of climate change. He said, "We should expect that what's happening on this mountain top is happening more generally in the Andes, and other tropical mountain ranges."

Teenage summer camps

    We helped on a nature reserve(保护区)for a week. We did different things every day, including cutting the reeds(芦苇),by the lake. Every day, we took turns to cook, in teams of five. When it was our teams turn, we made a simple dinner of pasta and salad for everyone. Judging by the fact that there was none left, we didn't do too badly!

-Adam

    We stayed with families who had kids our own age, and because they were on a mid-term break they came with us on all the trips. So except when we were actually in a language class, we were spending time with our new friends. It was a great way to practice the language we had been working on in the classroom.

-Oliver

    Every morning we had the same breakfast, and then cleaned up the camp. When it was all completely tidy, we could head off for the day. The first time we went into the city, we were put into teams and given lists of things to spot, like statues, squares and other landmarks. With some help from the locals, my team found almost all of the things on our list and came second. We went to different museums and galleries in and around the city every day. It was a great chance to learn about another country and its history.

-Sarah

    We stayed in a youth center in the suburbs, and went into the center of town by bus every day. It was a good way to see a bit of the city, and it didn't take long to get to the theater where we had our classes. I loved the atmosphere in the old building, and we could walk around during our breaks and watch rehearsals(排练). We learned a great deal and I'm looking forward to putting it all into practice at my school drama club next term!

-Malika