题目内容
（请给出正确答案）
[主观题]
______, the world may have population of 12 billion in a century.A．If man can live longer
______, the world may have population of 12 billion in a century.
A．If man can live longer in the future
B．If the science of medicine is developing fast enough
C．If the population growth is not reduced
D．If the earth is able to provide a good life for more people
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