Some teachers often give their students too much work to do, ________that they will thus improve

their school grades.

A. hoping  

B. to hope  

C. hope  

D. hoped

电力系统（直接接地系统）发生BC相短路，其边界条件为（）。

A.B=C=0，A≠0

B.A=B=C，A=B=C

C.A=B=C，A≠0

D.B=C=0，A=0