单选题 3.  They overcame all the difficulties and completed the task two months ahead of time, ______ is something we had not expected.
【正确答案】 A
【答案解析】 考查定语从句
   根据句意“他们克服了各种困难，终于提前两个月完成了任务，这是我们从未想到的”，需用which来指代前面整句话，引导非限制性定语从句，故选A。