单选题 They overcame all the difficulties and fulfilled the plan ten days ahead of schedule, ______ was something we had not expected.
【正确答案】 D
【答案解析】语法结构题。根据句法知识，此处应为一非限制性定语从句。故D为正确答案。句意：他们克服了所有困难并提前十天圆满完成了任务，这是我们没有预料到的。