单项选择题

The harm radiation has done to the Apollo crew members ______.

A．is insignificant
B．seems overestimated
C．is enormous
D．remains unknown
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单项选择题
It can be inferred from the passage that ______.
A．the Apollo mission was very successful
B．protection from space radiation is no easy job
C．astronauts will have deformed children or grandchildren
D．radiation is not a threat to well-protected space explorers
单项选择题
According to the first paragraph, the atmosphere is essential to man in that ______.
A．it protects tim against the harmful rays from space
B．it provides sufficient light for plant growth
C．it supplies the heat necessary for human survival
D．it screens off the falling meteors
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