Sorry, Cathy, I didn't know  ____   that you were badly short of money then. But youme for help. [ ]

A. must have asked            

B. could have asked

C. would have asked            

D. may have asked

当发电机在电力系统中并列运行时，改变其中一台发电机的励磁电流就会引起各并列的发电机间无功功率的（）。