The physicist has made a discovery, ______ of great importance to the progress of science and technology.

【正确答案】 C

【答案解析】逗号后为非限制性定语从句，修饰a discovery，用which连接，I think可视作插人语，故答案为C。引导词which在从句中可充当句子成分，D项中的it多余；that不能引导非限制性定语从句，B不正确。