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

一般来讲，预应力混凝土适用于以下哪些构件
Ⅰ．轴心受拉构件 Ⅱ．受弯构件
Ⅲ．轴心受压构件 Ⅳ．偏心受拉构件

A．Ⅰ、Ⅱ、Ⅲ
B．Ⅰ、Ⅲ、Ⅳ
C．Ⅱ、Ⅲ、Ⅳ
D．Ⅰ、Ⅱ、Ⅳ