— Look! The window is broken. Someone broke it _____ purpose.

— That may be right. But perhaps it was broken _____ accident. [ ]

A. on; by

B. by; by

C. on; on

D. by; on

可编程序控制器中软继电器的特点有（）。

A.体积小、功耗低

B.无触点、速度快、寿命长

C.有无数个常开、常闭接点供程序使用