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They overcame all the difficulties and fulfilled the plan ten days ahead of schedule, ____ was something we had not expected.
A、that
B、this
C、it
D、which
【正确答案】：D
【题目解析】：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。