通读下面的短文，掌握其大意，然后从各题所给的A、B、C三个选项中选出一个最佳答案。

Dear Sally,

    How are you? I'm happy that you can do the things you 1 at home. I never have fun, because there are too many 2 in my family. My parents are strict. I have to  3 at six o'clock every morning. And I have to make my bedroom4 before breakfast and then go to school. I can't play with my friends after school. I have to do 5 homework. After dinner I have to take the dog for    6 walk in the park. I can't watch TV on school nights. I have to  7 the violin for an hour. I can't go to bed late. I have to be in bed 8 nine o'clock. On weekends I have to clean my room and 9 the dishes. I also have to go to learn the violin on Sundays. I'm 10  tired. Can you help me?

                                                                                                                                                                    Yours,  

                                                                                                                                                                      Lisa

交流发电机正常工作时产生的电动势$e=E_m\sin \omega t$，若线圈匝数减为原来的一半，而转速增为原来的2倍，其他条件不变，则产生的电动势的表达式为（     ）

请计算：