He found          difficult to improve his English          the teacher’s help.

A．it is; without

B．it was; with

C．it; without

D．that is; with

"有效的控制系统应是合理的、适用的"体现的控制特征是（）

A.合理性

B.灵活性

C.适用性

D.战略高度

E.多重标准