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 spacesuit needn't worry about radiation damage

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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

A small population may mean ______.

A．higher productivity but a lower average income
B．lower productivity but a higher average income
C．lower productivity and a lower average income
D．higher productivity and a higher average income