题目

下列关于力的说法错误的是（      ） A．力的作用时相互的 B．支持力和压力的方向总是垂直于支持面 C．弹力、摩擦力是按性质命名的力 D．摩擦力的方向总是与物体运动的方向相反 答案：D --- Does your brother serve in the army?   --- No, not now. But he ________ in the army for 8 years.   A. would serve       B. served           C. had served       D. was serving