’t know _________. What can you suggest?
A. when                   B. how                     C. why                     D. where
【分析】此题应选 B，其余三项均有可能误选，其实此句为省略句，补完整应为 … but I don’t know how to improve my English.

请看以下类似试题：(1) I think the door is locked, but I’
　　答案选D，为 …and make sure it is locked. 之省略。
　　’t remember _________. A. where                  B. there                    C. which             D. that
答案选C，为I can’t remember which city he comes from. 之省略。

(3) Some of the answers were correct, but I don’t remember _________.
A. where                  B. what                    C. which                  D. that
答案选C，I don’t remember which 后省略了 ones (were correct)。

(3) He didn’t sell half as many computers as he thought he _________.
A. had                      B. was                      C. would                  D. sold
答案选C，为He didn’t sell half as many computers as he thought he would sell. 之省略。