Why was scientific satellite MAVEN sent to space?

In this section, you will hear five short news items. After each item, which will be read only once, there will be a twenty-second pause. During the pause, read the question and the four choices marked A, B, C and D, and decide which is the best answer. Then mark the corresponding letter on the answer sheet with a single line through the centre.
16. In September, the U. S. scientific satellite MAVEN(Mars Atmosphere and Volatile Evolution Mission)entered an orbit around Mars, with the mission of learning what happened to the Red Planet’s atmosphere and water. MAVEN’s principal investigator Bruce Jakosky says scientists now know that Mars once had a much denser atmosphere and that it has changed significantly o-ver the last few billion years. What they still don’t know is how and why that happened.
17. Scientists have been experimenting with four-legged robots for years, trying to see if they could be used as pack animals for carrying heavy loads over a difficult terrain. But the machine’s power requirements limited their potential. That’s changed, with a running robot designed by a group of engineers at Massachusetts Institute of Technology that operates on batteries rather than heavy gasoline-powered engines.
18. People often blame Hollywood movies, U. S. television shows and fast food for spreading American culture around the world. But a college entrance exam? Some people are criticizing planned changes to the Scholastic Aptitude Test as an attempt to spread American values to students o-verseas. Starting in 2016, the SAT will require test-takers to read from parts of the U. S. Constitution, the Declaration of Independence or other historical documents.
19. The cycle of poverty is often difficult to break, especially for women. Author Ritu Sharma witnessed the difficulties first-hand in travels through four countries including Sri Lanka. She shares her experience in her book Teach a Woman to Fish. Sharma explains to Daybreak Asia’s Frances Alonzo ways she sees that women can challenge and break free from oppressive systems that keep them poor.
20. The U. S. holiday shopping season is officially under way, with Black Friday discounts drawing millions of shoppers in stores and online. From California to New York—this is how the holiday shopping season begins—the early lines, the massive crowds—many of them lured in by Black Friday specials, from $ 99 flat screen TVs to toys at less than half their usual price. It’s enough to bring in the savvy shoppers and those just curious to see what all the fuss is about.