问题补充：
There are so many kinds of MP3 in the shop. We can’t decide .
A. what to buy
B. to buy what
C. which to buy
D. to buy which
答案：
A. what to buy
B. to buy what
(答案→)C. which to buy
D. to buy which
解析：原句意思是：商店里有这么多种MP3播放器。我们决定不了要买哪一个。选项A. what to buy买什么（指在不同种类中选）；B. to buy what买什么（词顺序有错，不合语法）；C. which to buy要买哪一个（指在所有的选项中选择买哪一个）；D. to buy which买哪一个（词序有错，不合语法）。根据句意答案选C。
