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

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

A．is insignificant
B．seems overestimated
C．is enormous
D．remains unknown

单项选择题

One of the difficulties in carrying out worldwide birth control is that ______.

A．official attitudes to population and birth control vary from country to country
B．different countries have different customs and practices on birth control
C．decreasing birth rate may lead to unemployment in some countries
D．in some countries people think that they enjoy the right to have as many children as they like