单项选择题 滚动摩擦系数和滑动摩擦系数都没有单位。

单项选择题 VIN码由（   ）位字符组成，第(   )位代表生产年份。A. 17、10B. 17、9C. 18、10D. 18、9

单项选择题 3) “People who live along this road receive their mail in these boxes.”   “Why are all of the____ ?” A、mailboxes painted gray  B、mailboxes that painted gray C、gray painted mailboxes   D、mailboxes painted as gray