— Must I return this book today, Mrs. Smith?    

— No, you ______. You may keep it until next week. [ ]

A. may not

B. can't

C. must not

D. needn't

如图所示，A，B两个滑轮中，A是______滑轮，B是______滑轮，在不考虑滑轮重和摩擦时，物体与桌面的摩擦力是90N，匀速移动物体，水平拉力F为______N．