一氧化氮（NO）为大气污染物，但少量一氧化氮在人体内，具有扩张血管、增强记忆功能．一氧化氮密度比空气大，难溶于水，极易与氧气反应．若让你收集一瓶一氧化氮气体，应选用的收集方法是（ ）
A．向下排空气法
B．向上排空气法
C．排水集气法
D．排水集气法或向下排空气法
NO难溶于水，可用排水法收集；密度比空气的略大且极易和空气中的氧气反应，故不能用排空气法收集；
故选C．
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| Long long ago, there was a swan (天鹅) with golden feathers (金羽毛). She lived in a lake. A woman lived in a small house 1 the lake with her two daughters. They were very poor. They worked hard all year round, 2 still, they lived a hard life and sometimes they even didn't have enough 3 to buy food. The swan was 4 to see that. She said to herself, "I'll give one of my 5 to them each day, then they can live a happy life with the money selling my feathers. "That evening, she 6 to the poor woman's house and left a golden feather on the table without saying 7 . From then on, the swam came every day and gave them a feather. The woman was very happy because their life was much 8 than before. But day after day,t he woman became greedy (贪婪). She said to her 9 . "The swan may fly away one day. If so, we will be poor again. We should take all 10 feathers shen she comes next time." "Oh, no, Mom!" cried the daughters, "This will 11 the swan. She helps us a lot! "But the mother wouldn't listen. When the 12 came as usual,t he mother caught her and took all her feathers. But suddenly, the golden feathers 13 chicken feathers. Then, the Golden Swan said, "Poor Mother, I came to 14 you, but you wanted to kill me. Now, I am leaving and will 15 come back. Never be greedy!" With these words, the swan flew away. | ||||
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