题目

I insisted that he severely and a doctor immediately. A . should be wounded; was sent for B . had been wounded; sent for C . had been wounded; be sent for D . be wounded; be sent for 答案：C 关于产生摩擦力的条件，下列说法中正确的是 A．相互压紧的粗糙物体之间总有摩擦力存在 B．相对运动的物体间一定有滑动摩擦力存在 C．只有相互挤压和有相对运动或相对运动趋势的粗糙物体之间才有摩擦力的作用 D．只有相互挤压和发生相对运动的物体之间才有摩擦力的作用