计算机要运行磁盘上的程序时，应先把程序文件读入到（）。

A、硬盘

B、ROM

C、RAM

D、CPU

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