The chief reason for the population growth isn’t so much a rise in birth rates a fall in

问题补充：

The chief reason for the population growth isn’t so much a rise in birth rates a fall in death rates as a result of improvements in medical care.

A. and

B. as

C. but

D. or

答案：

A. and

(答案→)B. as

C. but

D. or

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