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
According to the first paragraph, the atmosphere is essential to man in that______.
A．it protects him 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
单项选择题
We know from the passage that______.
A．exposure to even tiny amounts of radiation is fatal
B．the effect of exposure to radiation is slow in coming
C．radiation is avoidable in space exploration
D．astronauts in space suits needn't worry about radiation damage
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