He has made another wonderful discovery I think is of great importance to science. A. wh

问题补充：

He has made another wonderful discovery, I think is of great importance to science.

A. which

B. what

C. when

D. that

答案：

(答案→)A. which

B. what

C. when

D. that

解析：考察定语从句。Which指代先行词discovery在句中作主语。What不是定语从句的关系词，when是关系副词，that不能引导非限制性定语从句。故A正确。