专作驱动用的驱动绳轮和摩擦卷筒,它们都是利用()来驱动绳索的。
参考答案:摩擦力
— Did Tom return your book?
— Yes, but he _____ it for ten days before he returned it. [ ]
A. had kept
B. has kept
C. had borrowed
D. has borrowed
打卤面是()的特色面。