单项选择题

The best title for this passage would be ______.

A．The Atmosphere and Our Environment
B．Research on Radiation
C．Effects of Space Radiation
D．Importance of Protection against Radiation
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单项选择题
To food a large population, ______.
A．inferior land must be cultivated
B．good land must work intensively
C．each person must produce less
D．Both A and B.
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
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