呕血最常见的原因为（）。

A.急性胃黏膜病变

B.食管静脉曲张破裂

C.消化性溃疡

D.食管贲门黏膜撕裂

E.胃癌

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