题目

煤矿中瓦斯爆炸危害极大。某同学查资料得知含有瓦斯的气体的折射率大于干净空气的折射率，于是他设计了一种利用光的干涉监测矿井内瓦斯的仪器，其原理如图所示。在双缝前面放置两个完全相同的透明容器A和B，容器A与干净的空气相通，在容器B中通入矿井中的气体，让红光射入狭缝，观察屏上的干涉条纹，就能够监测瓦斯浓度。对于这个仪器的工作过程，下列说法中正确的是（ ） A . 如果屏的正中央仍是亮纹，说明B中的气体一定不含瓦斯 B 文中共有10处错误，每句中最多有两处。错误涉及一个单词的增加、删除或修改。增加：在缺词处加一个漏词符号∧，并在此符号下面写出该加的词。删除：把多余的词用斜线＼划掉。修改：在错词下面划一横线，并在该词下面写出修改后的词。注意：1、每处错误及其修改均仅限一词；2、只允许修改10处，多者从第11处不计分。Several American students visited our school. Upon our arrival, we gave a warm welcome to them, among them a boy named Tim was to stay with my family for the weekend. I recognized Tim compared him with photo he had sent to me. Tim greeted me and expressed that how excited he felt. When school was over, we took the subway home. My parents prepared a rich family dinner that night and we had enjoyed ourselves so much, eating and chatting. The next day, I act as a guide, showing Tim along the most famous scenic spots in Beijing. Wherever he went, he had some photos take to help him memorize this unforgettable trip to China. The time to say goodbye came when we realized it. I gave Tim a kite as a souvenir.