如图所示为一皮带传动装置，右轮半径为r，a点在它的边缘上．左轮半径为2r，b点在它的边缘上．若在传动过程中，皮带不打滑，则a点与b点的向心加速度大小之比为(        ) 

张叔叔到文具用品商店买了2枝钢笔，每枝钢笔18元。他付给营业员100元，应找回多少元？ 

国家能源局网站4月4日发布通知要求2014年全国淘汰煤矿1725处，淘汰煤炭落后产能11748万吨。对这一政策理解正确的是（）

    Mr. and Mrs. Su don't often go out in the evening, but today they go out for a walk after supper. They go to the park and have a good time there. On their way home, Mr. Su says to Mr. Su, "Look! A woman is running in the street and a man is running after her." Mr. Su says, "What's the matter? Let's go and help her."

    Now the woman gets near them. Mr. Su asks, "Shall we help you?" "No, thank you," the woman says, "My husband（丈夫） and I are running home, and the slower (较慢的) one does the housework."