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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

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

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

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

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