题目

放在光滑水平面上的物块1、2用轻质弹簧秤相连，如图所示，现对物块1、2分别施加方向相反的水平力F1、F2，且F1 >F2，则弹簧秤的示数          （    ）A．一定等于F1+ F2B．一定等于F1—F2C．一定大于F2小于F1D．条件不足，无法确定  答案：C解析:略 His grandfather __________ for thirty years.A. died B. was dead C. has been dead D. has died