题目

如图所示，一玻璃砖的横截面为半圆形，为截面的直径，是上的一点，且与点的距离（为半圆形截面的半径）。一束与玻璃砖横截面平行的白光由点沿垂直于的方向射入玻璃砖，从玻璃砖的圆弧面射出后，在光屏上得到由红到紫的彩色光带。如果保持入射光线和光屏的位置不变，而使玻璃砖沿向上或向下移动，移动的距离小于，则    （    ）A. 向下移动时，屏上红光最先消失  B. 向下移动时，屏上紫光最先消失C. 向上移动时，屏上红光最先消失  DOne terribly cold day, a snowstorm hit our area. Needing firewood, I quickly set out with my best friend, Bruce, to look for a tree to cut down. Cutting down a tree in a snowstorm can be dangerous. So it was important to have a friend who could warn me of dangers. When I chose a big 23-meter-tall tree and prepared to cut it down, Bruce suddenly warned me, “Don’t cut down that tree! It’s too close to the power line!” I wasn’t sure about it. So I decided to disregard his warning. I couldn’t wait to finish the job and go back home. I began cutting down the tree. When the tree fell, there was no longer any doubt whether my friend was right. The tree caught the power line, bringing it to the ground. I considered cutting the tree off the line. After all, electricity can’t travel through wood. As I reached out my finger to touch the tree, pain ran up my arm and through my head. I had been shocked! After I returned home and told my mother what happened, she quickly called the power company. Workers from the power company soon arrived. One of them asked if I had touched the tree, and when I told him I had, his face turned pale. “You should have been killed,” he said. It was my boots that saved my life. Within two hours, the workers removed the tree. Soon the snowstorm calmed down——but not my mother. Even though she was glad that I wasn’t hurt, my mother was still serious with me. After all（毕竟）, I shouldn’t have brushed off my friend’s warning. 1. Although Bruce warned him, the writer still cut the tree down because ____________.        A. he was sure there would be nothing to happen        B. he wasn’t sure if Bruce was right and expected to go home earlier        C. he didn’t hear his friend’s warning at all because of the blowing wind        D. he knew electricity can’t travel through wood 2. The underlined phrase “brushed off” in the last paragraph may mean “____________”.        A. pay no attention to     B. give up        C. think over         D. give in 3. Which of the following statements is NOT true according to the passage? A. Without his boots, the writer would have been killed by electricity. B. The writer’s mother was angry with what he had done. C. When the tree was cut down, the writer realized his friend was right. D. When the tree caught the power line, Bruce called the power company. 4. In the passage, the writer mainly wanted to tell us ____________. A. one of his terrible experiences when cutting down a tree B. not to cut trees down any more to prevent dangers from happening        C. it’s important to listen to other people’s opinions D. to be careful while we’re working with friends