某人用力F抬起放在水平地面上的一匀质杠杆AB的B端,F方向始终竖直向上,如图所示,则在抬起的过程中(? )A. F逐渐变大 B. F逐渐变小 C. F保持不变 D. 无法确定 答案:【答案】C【解析】图中某人用力F抬起放在水平地面上的一匀质杠杆AB的B端,F方向始终竖直向上,阻力的大小为杠杆的重力,整个过程中,阻力大小不变,根据数学知识可得动力臂与阻力臂的比值不变,由杠杆的平衡条件可得F的大小保持不变,答案选C。21.[1994上海] His wife had the front door painted green yesterday, ________ she?
A. did
B. had
C. didn't
D. hadn't