如图所示，一个单匝矩形线圈长为a，宽为b，置于磁感应强度为B的匀强磁场中，绕OO'轴以角速度匀速转动。从图示位置开始计时，经过时间t时感应电动势为（ ）
A. What do you want to be?
B. What subject do you want to teach?
C. Do you want to be a doctor?
D. I'm good at singing and dancing.
E. They help people.
Jerry: Hi, Linda. What do your parents do on Sunday?
Linda: They go to work on Sunday. They are doctors. ____
Jerry: ____
Linda: No, I don't.
Linda: I want to be a teacher. I want to teach children.
Linda: I want to teach Music. I like Music. ____
匀速转动。从图示位置开始计时，经过时间t时感应电动势为（ ）
取得最大值的最优解有无数个，则a为（ ）