题目

Mark Twain was a great writer. He was from the USA. He was born in 1835. He was also a famous speaker. He was famous for his sense of humour. Many people liked to listen to him talk because he liked to tell some interesting stories to make people laugh all the time. One day Mark Twain was going to a small town because of his writing. Before he was going to leave, one of his friends said to him that there were always a lot of mosquitoes in the town and told him that he'd better not go there. Mark Twain waved his hand and said, “It doesn't matter. The mosquitoes are no relatives of mi某无毒气体的密度约是空气密度的0.55倍，且极难溶于水，那么收集该气体的方法是（ ）①向上排空气法 ②向下排空气法 ③排水法．A.①②B.①③C.②③D.①②③