参考答案：B

解析：

本题测试虚拟语气：条件从句使用过去完成时，主句一般使用would have done。例如：I would have been glad if she hadn’t come this morning．如果她今天上午没来，我会很高兴的。The instructions would have been clear enough if only she had spoken in a more distinct voice．如果当时她用清晰的声音说出来，训令就不至于含糊不清。另外，虚拟语气条件状语从句可以将连词if省略，注意此时从句要使用倒装形式：Had he taken a little more time to think，he might have acted more reasonably．如果他再多用一点时间思考一下，他的行动就可能会更理智些。Should they come to us，we would ask them some questions．如果他们来我们这里的话，我们要问他们几个问题。Were I to do the work，I would do it some other way．如果我做这项工作，我就用别的办法。