流行病学实验选择对象时以下哪几点是正确的（）。

A.免疫力低的山区人群

B.预期发病率高的人群

C.选择能将实验坚持到底的人群

D.选择依从性好的人群

E.选择病情较重的人

The kinked demand model assumes that below the current price, the demand curve becomes:（）

A. less elastic because competitors will not decrease their prices.

B. more elastic because competitors will not decrease their prices.

C. less elastic because competitors will decrease their prices.