—          were electric slippers invented?  

—Sorry. I don't know.     [ ]

A. Who                      

B. Whom    

C. Of whom                  

D. By whom

国家放宽社会资本的投资领域，鼓励和引导社会资本以( )等方式，参与经营性的公益事业、基础设施项目建设。

A．贷款

B．独资

C．合资

D．合作

E．联营