题目

经研究发现，学生的注意力与老师的授课时间有关，开始授课时，学生的注意力逐渐集中，到达理想的状态后保持一段时间，随后开始逐渐分散．用f（x）表示学生的注意力，x表示授课时间（单位：分），实验结果表明f（x）与x有如下的关系：f（x）=． （1）开始授课后多少分钟，学生的注意力最集中？能维持多长的时间？ （2）若讲解某一道数学题需要55的注意力以及10分钟的时间，老师能否及时在学生一直达到所需注意力的状态下讲完这道题？I was sitting in a fast food restaurant with my two children, who at the time were about 6 and 9. Inside there was also a group of teens in Gothic(哥特式的) dress and appearance. Everyone kept away from them inside the restaurant.Outside the restaurant, people jumped past the homeless man searching the garbage bin, who was looking for food or perhaps a cigarette. I too had rushed past him with my youngsters, refusing his request for money but thinking maybe, just maybe, I would bring him some food. It turned out that I didn't have to.One girl in the Gothic youth jumped up, went to the counter and ordered more food, which she then took outside to the homeless man and sat on the bench with him, chatting away as he ate. He didn't say much—he was greedy—but I think I saw a bit of a smile and a flash in his eyes as he nodded to her questions.Writing this now, several years later, I still choke up. That girl was a model citizen living the values of goodness and kindness. These are values I have always held high and hoped to teach my kids. She made my job much easier. We stopped by on our way out and thanked her for setting such a positive example for us. I hope that hearing that has made as much of a difference in her life as seeing what she did has made to ours.1.Why did people keep away from the teens in the fast food restaurant?A. Because they were dressed in Gothic dress.B. Because they were too young to pay their meal.C. Because they liked to quarrel with others.D. Because they killed two children.2.What did the homeless man want the author to do?A. To help him get money. B. To bring him some food.C. To provide him with money. D. To refuse to take care of him.3.It can be inferred that the homeless man ________.A. was very greedy in deed B. was very hungry at that timeC. didn't like talking with others much D. didn't answer the girl's questions4.What can we learn about the author from the passage?A. The author is also a model citizen like the girl.B. The author sets a good example for the girl.C. The author is deeply moved by the girl.D. The author has sympathy for the girl.