Some of my friends are interested in science,but none of them can tell            .[ ]

A. when UFOs will appear next time

B. why do horses know the way

C. where was this kind of plant found

D. how do elephants communicate

评价预防接种效果的流行病学指标最主要的是（）

A.抗体水平

B.接种后反应率

C.安全性

D.保护率

E.发病率