achievement,last week's ministerial meeting of the WTO here earned a low, though not failing,

grade.    [ ]

A. In terms of      

B. In case of    

C. As a result of    

D. In face of

学生回答问题后教师没做任何评价，学生以后不再举手回答问题，这种现象是（）。

A.正强化

B.负强化

C.惩罚

D.消退