I've become good friends with several of the students in my school        I met in the English speech

contest last year.    [ ]

A. who      

B. where      

C. when    

D. which

动产质押合同自（）。

A.双方达成协议时生效

B.双方签订质押合同时生效

C.质物移交给质权人占有时生效

D.登记时生效