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
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
C