题目

I took the bus to work for many years.___________ knew each other; the passengers all sat there sleepily in the morning. The bus was cheerless and quiet.One of the passengers was a small old man _____________ gray hair. He took the bus to the center for the old every morning. None of us ever paid ______________ attention to him.One July morning he said good morning to the driver and smiled. The driver nodded without any expressions. The rest of us kept in ______________.The next day, the old man got on with a big smile again and said in a ___________ voice: And a very good morning to you all! 7．在测量滑轮组机械效率的实验时，小明同学利用如图所示的滑轮组提升重物．他做了24J的有用功使重物匀速上升了0.4m，则动滑轮重为15N（不计绳重及摩擦）；该滑轮组的机械效率为80%．（g取10N/kg）