单项选择题


A．source
B．origin
C．course
D．finance
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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．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
The harm radiation has done to the Apollo crew members ______.
A．is significant
B．seems overestimated
C．is enormous
D．remains unknown
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