— He works quite hard.

— Maybe you don't know that he always ____ without asking for leave when he ____ in another company. [ ]

A. went to work; was

B. goes to work; is

C. didn't go to work ; was

D. doesn't go to work; is

之所以要设置V/Hz限制，是因为（）或（）都将使工作磁通密度增加，工作磁通密度增加使励磁电流增加，特别是在（）的情况下，励磁电流将急剧增加。