单项选择题

Questions 17 to 20 are based on the following passage. At the end of the passage, you will be given 20 seconds to answer the questions. Now listen to the passage What is one of the things that must be done before man can live on Mars

A．Its surface must be studied.
B．Its temperature must be raised.
C．Big spaceships must be built.
D．Its air must be purified.

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单项选择题

单项选择题

Questions 17 to 20 are based on the following passage. At the end of the passage, you will be given 20 seconds to answer the questions. Now listen to the passage What is one of the things that must be done before man can live on Mars 您的浏览器不支持 audio 元素。