CO₂气体在一定条件下可与金属钠反应，干冰在一定条件下也可以形成CO₂气体，这两个变化过程中需要克服的作用分别是（）

    Ann is my classmate. She comes from New York, and she lives in Shanghai with her parents now. Every morning, she goes to KFC to have breakfast. Sometimes she buys a hamburger for me. She can't speak Chinese well. In the evening, her father often helps her with it. He's a teacher in our school. He can speak Chinese very well.

    Ann thinks the school life(生活)in China is very interesting. Ann likes reading Chinese books because she wants to know more about China. She tries(努力)to talk with her Chinese friends in Chinese. She hopes to speak Chinese well. She is very happy to live in China. She loves people here.

清前期《望江南百调》唱道：“扬州好，侨寓半官场，购买园亭宾亦主，经营盐、典仕而商，富贵不归乡。”材料反映了(    )

一片钥匙只能开一把锁，现有10片钥匙和10把锁，最多要试验（　　）次能保证全部的钥匙和锁匹配．