He has a good knowledge of electricity, ________ helps him a lot in his work．

A.that

B.who

C.when

D.which

答案及解析： D

解析：

该题考查定语从句中关系词的选用。此题没有先行词，整个主句“He has a good knowledge of electricity”是先行项，当整个主句作为先行项时，关系代词通常使用which，引导一个非限制性定语从句，which指的是整个主句。