Mr. Wells has found some new clues in his investigation, ______ of great importance to solve this criminal case.
【正确答案】 C
【答案解析】非限定性定语从句用法。在本句中，which引导了一个包含插入语I think的非限定性定语从句。A中which的位置不对，B中that不能引导非限定性定语从句，D中多了代词it。be ofimportance相当于be important。