单选题 My wallet is nowhere to be found. I ______ it when I was on the bus.
A. must drop B. had dropped
C. must have dropped D. should have dropped

【正确答案】 C

【答案解析】[解析] must have done用以推断过去某个时间或到说话时为止的一段时间内必定发生了某事。在这种句子中必须有一个表示过去某个时间的时间状语或根据上下文可以明确判定谈论的是过去。