交流电流表或电压表指示的数值是（）。
A.平均值
B.有效值
C.最大值
D.最小值
参考答案：B
— Mum, Must I do my homework first today?
— No, you _____. You _____ watch TV or play with your friends, too. [ ]
A. mustn't; may
B. can't; can
C. don't have to; can
D. needn't; must
