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 do5 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

— Must I finish the work tonight?

— No, you       . But you         finish it tomorrow.

已知$\left|\overrightarrow{a}\right|=3,\left|\overrightarrow{b}\right|=4$，且$(\overrightarrow{a}+k\overrightarrow{b})\perp (\overrightarrow{a}-k\overrightarrow{b})$，则k等于             (     )