Deserts are found where there is little rainfall or where rain for a whole year falls in only a few weeks' time. Ten inches of rain may be enough for many plants to survive if the rain is spread throughout the year. If it falls within one or two months and the rest of the year is dry, those plants may die and a desert may form.

    Sand begins as tiny pieces of rock that get smaller and smaller as wind and weather wear them down. Sand dunes (沙丘) are formed as winds move the sand across the desert. Bit by bit, the dunes grow over the years, always moving with the winds and changing the shape. Most of them are only a few feet tall, but they can grow to be several hundred feet high.

There is, however, much more to a desert than sand. In the deserts of the southwestern United States, cliffs (悬崖) and deep valleys were formed from thick mud that once lay under a sea more than millions of years ago. Over the centuries, the water dried up. Wind, sand, rain, heat and cold all wore away at the remaining rocks. The faces of the desert mountains are always changing –-very, very slowly —as these forces of nature continue to work on the rock.

    Most deserts have a surprising variety of life. There are plants, animals and insects that have adapted to life in the desert. During the heat of the day a visitor may see very few signs of living things, but as the air begins to cool in the evening, the desert comes to life. As the sun begins to rise again in the sky, the desert once again becomes quiet and lonely.

    Life may have evolved on at least three planets within a newly discovered solar system 39 light years from Earth. Astronomers at the NASA have discovered no less than seven roughly Earth-sized worlds orbiting a dwarf star in the system. Scientists had previously identified a tiny number of "exoplanets", which are believed to have the qualities needed to support life. However, the new system contains an unprecedented(前所未有) number of Earth-sized, probably rocky planets, and is being acknowledged as an "accelerated leap forward" in the search for extraterrestrial (外星球的)life.

    Three of the new planets are said to be particularly promising because there might exist oceans. Thomas Zurbuchen, associate administrator of NASA, told a press conference in Washington: "This gives us a hint that finding a second Earth is not a matter of 'if', but 'when'."

    The planets were discovered using NASA's Spitzer Space Telescope, which was able to track how fast each of the planets crossed the star Trappist-l and completed an orbit. From this the research team calculated how far the planets were from their star, and therefore how likely they are to be habitable.

    NASA's Hubble Space Telescope is already being used to search for atmospheres around the planets. Future telescopes, including the proposed European Extremely Large Telescope and the James Webb Space Telescope, may be powerful enough to discover markers of life, such as oxygen in the atmospheres of exoplanets.

    Astronomers believe there could be as many as 40 billion potentially habitable worlds in our galaxy, the Milky Way. Prof Zurbuchen said that now was a "gold-rush phase" in the search for these exoplanets.

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。

    More and more people like bicycling and it is no surprise. It is fun, healthy and good for the environment. Maybe that's why there are 1.4 billion bicycles and only 400 million cars on roads worldwide today. Bikes can take you almost anywhere, and there is no oil cost!

    Get on a bicycle and ride around your neighborhood. You may discover something new all around you. Stopping and getting off a bike is easier than stopping and getting out of your car.

    You can bike to work and benefit from the enjoyable exercise without polluting the environment. You don't even have to ride all the way. Folding (折叠) bikes work well for people who ride the train. Just fold the bike and take it with you. You can do the same on an airplane. A folding bike can be packed in a suitcase. You can also take a common bike with you when you fly. But be sure to look for information by getting on airline websites. Not all airlines are bicycle-friendly to travellers.

Health Benefits of Bicycling:

    It helps to prevent heart diseases.

    Bicycling helps to control your weight.

    A 15-minute bike ride to and from work three times a week burns off five kilos of fat in a year.

    Bicycling can improve your mood (心情).

    Exercise like bicycling has been shown to make people feel better, more relaxed and self-confident.

    Bicycling is healthier than driving.

    Winter sports tend to be more dangerous than summer sports — they take place mostly outside at the mercy of ice or snow, involving (牵涉) metal edges, extreme speeds and crashes. Skeleton (俯视冰梭), however, stands apart as one of the most difficult and least accessible of all the winter sports. Adam Edelman, Israel's first Olympic skeleton athlete, would like to change that.

    Edelman was the 10th athlete named to the 2018 Israeli Olympic Team. Qualifying for the Olympics in any sport is impressive. But it's more impressive if you qualify for the Olympics after only four years in the sport — and without a coach.

    Edelman was first introduced to skeleton watching the game on television in late 2013. Sports have always been a part of his life. He grew up playing football and was a goalkeeper on the Massachusetts Institute of Technology football team. Ready for a new challenge, he set his sights on the Olympics.

Without a coach, Edelman learned to drive by watching endless hours of YouTube videos and taking more runs than anybody else on the track. “When everybody else would take three runs per day, I would take six to eight.” he recalled.

    Succeeding in skeleton takes a large amount of athletic ability and technical skill, but there's one more element (因素) required, one that can't be taught. “These tracks but built in the middle of nowhere, and it's gray and cold. You haven't seen your family in seven months and you really just want to give up. To continue, it takes tenacity.” Edelman said.

    Edelman is already looking beyond Pyeongchang. The 2022 Olympics could be in his future, but whether he continues to compete or not, he promises to stay in Israeli sports as long as possible-as instructor or as a coach.

    Some people shy away from traveling alone while a few welcome it. So, what is it that makes traveling alone an attractive choice?

    First of all, the travel plan and all the decisions you make are yours alone. You don't need to worry about any other person or group. In other words, there are no arguments. Actually, you can be completely selfish.

    Traveling alone is also a great confidence builder. Yes, at times it can feel a bit lonely, but that is just one of many problems you will have to solve yourself, along with making your own preparations and setting your own goals.

    Besides, have you noticed that when you're on your own, people are more willing to start a conversation with you? Interestingly, people tend to keep a distance from couples and groups. However, the lone (独自的) traveler looks ready to connect with their fellow human beings and more likely to get into pleasant conversation about their trips.

    Here are two more exciting reasons why you should consider traveling alone. You will find you learn the language faster when you don't have someone else talking to you in your own language all the time. You are forced to communicate in the foreign language when you travel alone in a country that doesn't speak your native language. In addition, when you're on your own, you're free to meet someone who might turn out to be very important in your life.

    Of course, there may be things stopping you: You fear the unknown, or you have friends who may be upset by your decision to take off by yourself. You will have to make them believe the value of traveling done and allay any fears they might have, but with the technology at our fingertips, you can include them in the adventure.

    A European Union program is letting blind people experience famous paintings for the first time. It uses three dimensional (3-D) printing to re-create famous paintings so they can be touched.

    One painting printed with the new technology is Gustav Klimt's “The Kiss.” It is a popular attraction at the Belvedere Museum in Vienna, Austria. The painting shows a man and a woman standing in a field filled with flowers. They are wearing gold robes and have their arms around each other. The man leans down to kiss the woman.

    Klimt finished the painting in 1908. Until now, people who had trouble seeing could not appreciate the artwork. But thanks to the reproduction they can touch the piece and feel the ridges and depressions. Andreas Reichinger started making 3-D versions of artwork in 2010. He said this reproduction was his most difficult project because the couple's robes are so detailed.

    Dominika Raditsch is a blind museum visitor. She touched the reproduction. As she moved her hands around it she said, ”Exactly, can you see these? There are so many details.“ Raditsch said she can imagine what the original painting looks like when she touches the reproduction. “It's somehow round. You can feel it. It comes with it. And in many places it's so smooth. And then I think to myself: it probably shines too!” Raditsch said.

    The Belvedere is not the only museum to have 3-D versions of its artwork. Some of the pieces at the Prado, in Madrid, Spain, have reproductions that can be touched. But the piece in Vienna has one special part: it is made with widely available 3-D printing technology. That means one day, blind art fans anywhere in the world could download the source files and print the reproductions themselves.