问题
选择题
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
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。