I just don't understand_____________ that prevents so many Americans from being as happy as one might expect.

A. why it does                  B. what it does   

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.

People have come to understand _____ prevents so many Americans from  being as happy as one might expect.

A．that it is why       B．what does it that    C．why it does that     D．what it is that

I try to understand __________ that prevents so many Chinese from being as happy as one might expect.

A. why is it        B. what is it      C.why it is       D.  what it is

I just don't understand_______ that prevents so many Americans from being as happy as one might expect.

A. why it does       B. what it does       C. what it is            D. why it is