单选题 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

【正确答案】 B

【答案解析】[解析] not so much a rise in birth rates as a fall in death rates并非是(由于)出生率的增长，而是(由于)死亡率的下降。not so much A as B是个很有用的句式，还可以把这个句式改写为not A but B或B rather than A。