已知三相交流电的u=u_msinωt，u_m=311V，f=50Hz，则有效值为（）。

A.311V

B.220V

C.110V

D.180V

A: What shall I do I’ve got so many things on my mind now. Will anybody help me
B: __________________

A. How can I get support from others
B. Don’t expect me to help you. Is there anybody who can help me
C. Don’t ask me. It’s your baby, Mary. See, my hands are full.
D. Sorry. I didn’t expect you are so busy.