题干

“魔棒点灯”趣味实验的操作为:取少量高锰酸钾晶体放在表面皿上,在高锰酸钾上滴加2﹣3滴浓硫酸,用玻璃棒蘸取后去接触酒精灯的灯芯,酒精灯立刻被点燃了.你认为酒精灯能被点燃的主要原因是(  )

A:“魔棒”具有神奇的功力使酒精灯点燃

B:高锰酸钾与浓硫酸的混合物能降低酒精的着火点,使酒精燃烧

C:它们之间发生化学反应,放出大量的热,使温度达到酒精的着火点

D:实验者有特异功能,使酒精自发燃烧

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答案(点此获取答案解析)

C

同类题5

阅读下列四篇短文,从每题所给的A、B、C、D四个选项中,选出最佳选项。

    I had worried myself sick over Simon's mother coming to see me. I was a new teacher, and I gave an honest account of the students' work. In Simon's case, the grades were awfully low. He couldn't read his own handwriting. But he was a bright student. He discussed adult subjects with nearly adult comprehension. His work in no way reflected his abilities.

    So when Simon's mother entered the room, my palms (手掌心) were sweating. I was completely unprepared for her kisses on both my cheeks. “I came to thank you,” she said, surprising me beyond speech. Because of me, Simon had become a different person. He talked of how he loved me, he had begun to make friends, and for the first time in his twelve years, he had recently spent an afternoon at a friend's house. She wanted to tell me how grateful she was for the self-respect I had developed in her son. She kissed me again and left.

I sat, stunned, for about half an hour, wondering what had just happened. How did I make such a life-changing difference to that boy without even knowing it?What I finally came to remember was one day, several months before, when some students were giving reports in the front of the class, Jeanne spoke quietly, and to encourage her to raise her voice, I had said, “Speak up. Simon's the expert on this. He is the only one you have to convince, and he can't hear you in the back of the room.” That was it. From that day on, Simon had sat up straighter, paid more attention, smiled more, and became happy. And it was all because he happened to be the last kid in the last row. The boy who most needed praise was the one who took the last seat that day.

    It taught me the most valuable lesson over the years of my teaching career, and I’m thankful that it came early and positively. A small kindness can indeed make a difference.