单选题 11.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】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；B rather than A。