There might be as many as 10 million species of complex life on this planet today —— a huge number. But add up all of the complex species that ever lived and some biologists think the grand total would be about five billion. The estimate leads to an astonishing conclusion: a staggering 99% of species are not around any more. They have been driven to extinction.

    More species are joining the ranks of the extinct every year. Many scientists believe we are living through an episode of remarkably rapid extinction, on a scale that has been seen only five times in the last half a billion years.

    They call this current episode the sixth mass extinction —— a large, global decline in a wide variety of species over a relatively short period of time. And they tend to agree that humans are the main cause.

    Over-hunting, overfishing, and human-driven habitat loss are pushing many species to the brink. In fact, we have changed the planet so much that some geologists are now suggesting that we have entered a new phase in Earth's history; an epoch they call the ＂Anthropocene＂. By 2100, it is expected that humans will have caused the extinction of up to half of the world's current species.

    Because we are living through this extinction, it is relatively easy for us to study the driving forces behind it. But how do we determine what caused other mass die-offs that happened long ago? To do so we have to look at what archaeologists, palaeontologists, geologists and other scientists have concluded from the evidence they have gathered.

    The trouble is, those scientists do not always agree with one another —— even about the most recent extinction event. As well as the five-or six- mass extinctions, there have also been many smaller extinctions.

    One of these mini extinction events happened towards the end of the Pleistocene, a few tens of thousands of years ago. It is sometimes called the ＂megafaunal＂ extinction because many of the species it claimed were particularly large animals, weighing more than 97lb (44kg). However, its cause remains a debate amongst scientists.

    As we all know, friendship is important in our daily life. A friend can give us help and share our difficulties and happiness. But how to make friends is still difficult to some of us. {#blank#}1{#/blank#}

Be friendly to others. {#blank#}2{#/blank#} You should try to make a stranger feel at home wherever he happens to be. Think more of others than of yourself and never judge a person by his look and clothes.

    Be reliable. If you and your friend agree to meet somewhere, don't be late. If you're not going to make it on time or make it at all, call them as soon as you realize it. {#blank#}3{#/blank#} Be someone that people know they can depend on.

    {#blank#}4{#/blank#} Listen carefully to what people say, remember important details about them （their names, their likes and dislikes）, ask questions about their interests, and just take the time to learn more about them.

    Encourage your friends. {#blank#}5{#/blank#} A friend to all is a friend to none. Never make fun of or laugh at your friends in front of others. You should come to support them.

A.  Be a good listener.

B.  When you say you'll do something, do it.

C.  A true friend always gives courage to his friends.

D.  Never leave your friend when he is in trouble.

E.  The following information can give you some tips.

F.  Smile at others and you are sure to get a smile in return.

G.  He will remain with them in both good as well as bad times.

    We can video chat with astronauts aboard the International Space Station and watch live footage from the frozen heights of Everest. But communicating with a submarine (潜艇）or a diver is not so easy. The lack of practical methods for sharing data between underwater and airborne devices has long been a frustration for scientists. The difficulty stems from the fact that radio signals work perfectly in air travel but poorly in water. Sonar (声呐）signals used by underwater sensors reflect off the surface of the water rather than reaching the air.

    Now, researchers at MIT have developed a method with the potential to revolutionize underwater communication. "What we've shown is that it's actually feasible to communicate from underwater to the air," says Fadel Adib, a professor at MJT's Media Lab, who led the research.

    The MIT researchers designed a system that uses an underwater machine to send sonar signals to the surface, making vibrations (震动)corresponding to the ls and Os of the data. A surface receiver then reads and decodes these tiny vibrations. The researchers call the system TARF. It has any number of potential real-world uses, Adib says. It could be used to find downed planes underwater by reading signals from sonar devices in a plane's black box and it could allow submarines to communicate with the surface.

    Right now the technology is low-resolution. The initial study was conducted in the MIT swimming pool at maximum depths of around 11 or 12 feet. The next steps for the researchers are to see if TARF is workable at much greater depths and under varying conditions—high waves, storms, schools of fish. They also want to see if they can make the technology work in the other direction— air to water.

    If the technology proves successful in real-world conditions, expect "texting while diving" to be the latest underwater fashion.

    If you've been in public in the past year or so, chances are you've noticed someone breaking dining etiquette (礼仪) that would seem second-nature to any adult. Why does our dining etiquette seem to be at an all-time low right now.

    "Overall, there is much less feeling about the common good, which means caring about your neighbor—whether at home or in a restaurant," said Steven, who writes articles for USA Today.

    Take the use of cellphone at the table for example. While a 2015 survey by the Pew Research Center found that 38 percent of those surveyed thought it was acceptable to use their phones while dining out, the increasing number of cellphone bans in restaurant shows that restaurants aren't putting up with it.

    However, some look on the growing use of cellphones as a sign that manners change alongside our society.

    "Manners change," said the writer behind a food site The Takeout. "What we consider improper isn't set in stone." She points out the other "rules" that have since become outdated nowadays, like saying "Sir" and "Ma'am," as proof of this change.

    Yes, traditional manners are yet another thing being redefined by youth. "Younger people are growing up in a world with more flexible rules, and this spreads to the dining table," said Daniel Levine, director of global trends consultancy The Avant-Guide Institute.

    Whether you prefer high-end restaurants or a fast-casual meal, the biggest reason behind the decline of manners may just be a numbers game.

    "I believe it is more likely for people to break dining 'rules' because they go out and eat more often," said Diane Gottsman, owner of The Protocol School of Texas. "Years ago, people stayed home and sat around the family table. Today, there are more people in the workforce, which makes it more affordable for socializing and eating out. With new chance comes the possibility for more dining problems."

High on the Qinghai-Tibet Plateau {#blank#}1{#/blank#}(lie) China's "water tower" Sanjiangyuan. It is home to the headwaters of China's three rivers: the Yellow River, the Yangtze River, and the Lancang River. It is the three rivers {#blank#}2{#/blank#} provide water for about half the population of China. However, human activities are putting this ecosystem {#blank#}3{#/blank#} risk. Hopefully, though, we can repair this damage. In 2016, the Chinese government established Sanjiangyuan National Park in Qinghai Province. The park, {#blank#}4{#/blank#}(schedule) to open in 2020, has increased the area's green coverage and attracted more wildlife. The local government has also made great efforts to improve biological diversity. The success of Sanjiangyuan will mark the start of a {#blank#}5{#/blank#}(green) future. With Sanjiangyuan {#blank#}6{#/blank#}(take) the lead, nine other regions in China{#blank#}7{#/blank#}(choose) by a field conservation station to carry out projects over the past three years, {#blank#}8{#/blank#} will form a national park system together. The idea of a national park system is still new in China, but {#blank#}9{#/blank#}(it) realization is urgent. After all, building an ecological civilization bears great {#blank#}10{#/blank#}(important) in the development and progress of human civilization.

Last year, I baked biscuits for complete strangers to say "thank you". I'd had to call 999 because I found my husband unconscious on the floor. Within minutes, a police car arrived and soon my husband received medical care in hospital.

A week later, when I dropped off still-warm biscuits and presented a thank-you note at the police station, the policemen thanked me for delivering gifts.

I drove away feeling light and happy. Later, I realized that my natural high might have been more than it seemed. Research has shown that sharing gratitude has positive effects on health. People who express gratitude will increase their happiness levels, lower their blood pressure and get better sleep.

What about people who receive gratitude? Research has confirmed that when people receive thanks, they experience positive emotions. "Those are happy surprises," says Jo-Ann Tsang, a professor of psychology. When someone is thanked, he's more likely to return the favouror pass kindness on, and his chances of being helpful again doubles, probably because he enjoys feeling socially valued.

The give-and-take of gratitude also deepens relationships. Studies show that when your loved ones regularly express gratitude, making you feel appreciated, you're more likely to return appreciative feelings, which leads to more satisfactory in your relationships.

Nowadays, however, many people don't express gratitude. Our modern lifestyle may be to blame. With commercial and social media, everything is speeding the younger generation to feel they're the centre of the world. If it's all about them, why thank others?

Why not thank others? Just take a look at how many positive effects can saying "thank you" have on personal health—and the well-being of others.

If you aren't particularly grateful, I strongly suggest you learn to be. People who are instructed to keep gratitude journals, in which they write down positive things that happen to them, cultivate（培养） gratitude over time.