题干

如图是吊车起吊货物的结构示意图,伸缩撑杆为圆弧状,工作时它对吊臂的支持力始终与吊臂垂直,使吊臂绕O点缓慢转动,从而将货物提起.下列说法正确的是(   )

A:汽车轮胎有花纹是为了减少行驶过程中的摩擦力

B:汽车固定后,起重物上升过程中,对地面的压强变小

C:匀速顶起吊臂的过程中,伸缩撑杆支持力的力臂变小

D:匀速顶起吊臂的过程中,伸缩撑杆支持力渐渐变小

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答案(点此获取答案解析)

D

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    “Get up, get up, Jim. It's time to get up!” My mother said while she was pushing me. I opened my eyes and looked at the clock. “Oh dear! It's a quarter past eight already.” I'll be late for school again,” I thought. I jumped out of the bed, washed my face, and then hurried to the bus stop without breakfast. As soon as I   got on the bus, it started running. “I'm sure I can get to school in time,” I said to myself. Suddenly the bus stopped. The bus driver got off the bus, then got on the bus again and said, “I'm sorry, ladies and gentlemen. There's something wrong with the bus. Please get off the bus and wait for another one.” “I couldn't wait. The school isn't far. I'd better run to school,” I thought, and then I began to run. But before I got to school, it began to rain hard.

    I reached the classroom but I was wet shivering with cold. The math teacher had already begun his lesson. It was Mr. Smith. He was very strict with his students and the students were a little afraid of him and so was I. I liked neither him nor his math lessons. I hesitated (犹豫) for a moment, and then I opened the door. “I'm very sorry, Mr. Smith. I'm late again,” I said with a red face.

    “It's you again, Jim,” Mr. Smith said angrily, “I've told you not to be late, but you...” he suddenly stopped and looked at my wet clothes. He turned to   the class and said, “I'm very sorry, boys and girls. ” With these words he quickly took off his own jacket and handed it to me. “Now, take off your wet coat and put on this one or you'll catch cold,” he said kindly.

    I went to my seat with Mr. Smith's jacket on. It was a bit larger for me but I felt very warm in it. I found, for the first time, math was so interesting and how well I could understand it!

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