We are not conscious of the extent of which work provides the psychological satisfaction that can make the difference between a full and empty life.()

A. to which

B. in which

C. at which

D. by which

社会实践中学生所学的知识以（）的知识为主。

A．理论、系统

B．间接、抽象

C．概括、精炼

D．直接、感性