【问】 They were expecting the surgeon to finish the operation____dawn.

A. before B. till C. on D. in

介词before与介词till都表示“在……之前”，上道题除选A(before)外。能否还可选B(till)?

【答】 不能选B(till)。因为介词till(until)，如同连词till(until)一样。在肯定句中，它只能与持续动词(durative verbs)即持续一段时间的动词(如be，live，like，work，camp等)连用，而且表示“一直持续到……为止”。请对比下列例句：

1. My girlfriend worked there till Christmas. (CGEL)

2. They talked till dawn. (Lewis)

3. We must wait till Monday. (Gronin)

4. The ceremony will take place before the meeting. (CGEL)

5. I have to leave before midday. (CGEL)