Facebook is an Internet-based social network site that lets people get in touch with family and friends and reach out to people with common interests around the world, all through computer. It is very popular, with more than 900 million users. If Facebook were a country, it would have about three times the population of the United States.

    Documents filed with US. financial regulators say Facebook has hundreds of millions of active users who send billions of messages each day and upload 250 million photographs on their personal pages. Facebook users have registered 100 billion “friends.”

    On Facebook, “Friending” someone means you add the person to your list of people you can communicate with directly, which often allows the person to see more information about you than you share with the general public.

    Facebook was started by Mark Zuckerberg and other students at Harvard University in 2004, and has grown at an amazingly fast pace.

    Facebook makes money when advertisers pay to get access to hundreds of millions of Facebook users. Advertisers can often direct their messages to the people most interested in their products because Facebook computers keep track of information that users place on their pages. For example, a person interested in diving, planning a wedding, looking for work, or suffering from diabetes(糖尿病) may see advertisements related to those topics.

    Facebook has become so much a part of the culture of the United States and other nations that it has been the subject of a Hollywood movie. It is a key marketing tool for many businesses, and its users' content has been used as evidence in some divorce or criminal cases.

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。

    A recent study surveyed 5,000 British families about their experiences with volunteering and their mental health. Participants completed the same survey every two years from 1996 to 2008.

    About 20 percent of the survey participants reported doing regular unpaid work Researchers found that these volunteers also scored highest on their mental health scores. The two were linked so directly that the more a person volunteered, the happier they were.

    One could argue a chicken-and-egg theory: happier people are more likely to have the time, money, energy and resources to give back to others. But even when researchers adjusted for education, social class and total health, the link was there, suggesting that the mental increase came from volunteering and not the other way around.

    According to Dr. Stephen G. Post, author of The Hidden Gifts of Helping, when you help others, your brain releases feel-good chemicals, which can help cells repair themselves and grow. And this translates to better health.

    So volunteering can help improve both your physical and mental health—on one condition. You have to mean it. People who volunteered for "self-oriental" reasons, either because they were forced to do so for work or school or because they were trying to get away from problems in their lives, had a mortality（死亡率）risk that was similar to those who didn't volunteer at all. It was only when people were volunteering out of a true sense of sympathy that they saw any health benefits.

    So whether it's arranging books at the library or walking dogs at the animal shelter, find a cause that's important to you and give it a try. Volunteering is a good way to develop a sense of connection that not only helps your community, but may also be good for you.

    China now has 52 sites included on the UNESCO World Heritage List, which was established to safeguard unique and irreplaceable cultural and natural sites around the world. Let's have a look at some of them, which may give you some clues on your next travel plan.

    Mausoleum of the First Qin Emperor

    The Mausoleum of the First Qin Emperor is located in Lintong county, near Xi'an city, capital of Shaanxi province. A total of 8,000 life-size terracotta warriors, chariots and horses, and hundreds of bronze weapons have been unearthed from it. Many state leaders and experts worldwide believe that the discovery of the mausoleum is a major archeological find and can be called the eighth wonder of the world, paralleling Egypt's pyramids.

    Kulangsu, a Historic International Settlement

    Gulangyu Island, known in the local dialect as Kulangsu, is located on the entry of the Jiulong River, facing the city of Xiamen. The island was occupied by at least a dozen countries, including Great Britain, France and Japan. As a result, Western culture grew rapidly there in the 19th century, especially music. With such a harmonious blend of Western music and Eastern culture, Gulangyu is naturally referred to as the "island of music". It is famous for its outstanding musicians and musical instruments.

    Qinghai Hoh Xil

    Hoh Xil, which means "beautiful girl" in Mongolian, is located in the north-eastern end of the Qinghai-Tibetan Plateau. Hoh Xil National Nature Reserve is China's largest and the world's third largest uninhabited area. Because of the high altitude and fierce weather, it is impossible for people to live there, and therefore, it is a paradise for wildlife and home to more than 230 species of wild animals including endangered species such as the Tibetan antelope, which is recognized as the National Class I Protected animal.

    Huangshan Mountain

    Huangshan, known as "the loveliest mountain of China", lies in the south of Anhui province in eastern China. The scenic area covers 154 sq km, and is famous for its peaks, rocks, pines, clouds and springs. It also has a rich cultural heritage. Xu Xiake, a famous traveler of the Ming Dynasty (1368-1644), visited Huangshan twice, and described it in a poem, calling it the best of all mountains in China. Today it holds the same fascination for visitors, poets, painters and photographers.