题目

一辆汽车沿着平直的道路行驶，遇有紧急情况而刹车，刹车后轮子只滑动不滚动，从刹车开始到汽车停下来，汽车前进12米。已知轮胎与路面之间的滑动摩擦因数为0.6，（g取10m/s2） 求：刹车前汽车的行驶速度。 答案：12 m/sLinda was busy when we went to see her yesterday. She ________ for an exam. [　　] A．studiedB．was studying C．had studiedD．has studied