The team overcame all the difficulties and completed the task three weeks ahead of time, ______ is something we had not expected.
【正确答案】 B
【答案解析】分析句子结构可知，逗号之后的句子为非限制性定语从句，空处指代前面整个句子的内容，所以应用关系代词which引导，在从句中作主语。故选B。