观察、归纳与比较是学习化学的一种重要方法．氧气是一种化学性质比较活泼的气体，它能与许多物质发生化学反应，如氧气与硫、磷、铁发生化学反应的文字表达式为：
①硫+氧气
二氧化硫；②磷+氧气
五氧化二磷；③碳+氧气
二氧化碳．
通过分析、比较我们发现，它们有许多共同点，请写出其中两条：
①_________；②_________．
①都是化合反应；②生成物都是氧化物
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| Recently, one of my best friends, whom I've shared just about everything with since the first day of kindergarten, spent the week- end with me. Since I moved to a new town several years ago, we've both always 1 the few times a year when we can see each other. Over the 2 , we spent hours and hours, staying up late into the night, talking about the people she was 3 around with. She started telling me stories about her new boyfriend, about how he experimented with 4 and was into other 5 behavior. I was blown away! She told me how she had been 6 to her parents about where she was going and even sneaking out to see this guy because they didn't want her 7 him. No matter how hard I tried to tell her that she 8 better, she didn't believe me. Her self-respect seemed to have disappeared. I tried to 9 her that she was ruining her future and heading for big trouble. I felt like I was getting 10 . I just couldn't believe that she really thought it was 11 to hang with a bunch of losers, especially her boyfriend. By the time she left, I was really worried about her and 12 by the experience. It had been so frustrating, I had come 13 to telling her several times during the weekend that maybe we had just grown too far apart to 14 our friendship-but I didn't. I put the power of 15 to the ultimate test. We'd been friends for far too long. The chance was that she 16 me enough to know that I was trying to save her from hurting herself. I wanted to believe that our friendship could 17 anything. A few days later, she called to say that she had thought long and hard about our 18 , and then she told me that she had 19 with her boyfriend. I just listened on the other end of the phone with tears of joy running down my face. It was one of the truly 20 moments in my life. Never had I been so proud of a friend. | ||||
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