问题：
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 does it
B．what does it
C．what it is
D．why it is
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 does it | B．what does it | C．what it is | D．why it is |
参考答案：