我国古代,经常用“汗青”指代()?
参考答案:史册
— 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.