When I try to understand _______ that prevents so many Americans from being as happy as one might expect, it seems to me that there are two causes.

A．why it does

B．what it does

C．what it is

D．why it is

什么时期居住在高氟区可患氟牙症（）

A.7岁之前

B.7～12岁

C.12～15岁

D.15岁之后

E.任何年龄