1.__1__dull he may be, he is certainly a very successful top executive.
2.If only I__2__play the guitar as well as you!
3.The party,__3__I was the guest of honour, was extremely enjoyable.
A. by which
B. for which
C. to which
D. at which
4.It’s high time we __4__, cutting down the rainforests.
B. had to stop
C. shall stop
5.The student said there were a few points in the essay he__5__impossible to comprehend.
A. has found
B. was finding
C. had found
D. would find
【详细解答】选However。容易误选As，因为as有时用来表示“尽管”，但在那个句型中句首得是形容词或动词，比如可以说Dull as he may be, …。也可以把as放在句首，但要说成As dull as he may be, …。题干中说he may be而不说he is，有增加题目难度方面的考虑。
【详细解答】选at which。这道题出得意义不大，其实就是在考guest和party 之间选用什么介词。说at which显得最为自然。
【详细解答】选stopped。也可以说It’s time we stopped…。