题目

. They want to employ 90 people, __________last year.        A．three times as many as    B．more than three times        C．as three times as             D．three times that they did 答案：A用如图所示的滑轮组，匀速水平向左拉重为100N的物体A，所用拉力F为10N（不计滑轮与绳重及滑轮与绳之间的摩擦），则A受到地面对它的摩擦力大小是________N．