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阅读以下材料,回答问题.

公元前213年,罗马帝国派出大批战船开往地中海的西西里岛,想征服叙拉古王国.

这一天,晴空万里,阳光灿烂,阿基米德和叙拉古国王站在城堡上观察着海面.远处那一只只露出一些桅顶的罗马战船慢慢地越变越大.

城堡中兵力很少.国王把希望目光投向聪慧无比的阿基米德,询问道:“听说您最近叫人做了很多的大镜子,这里面有些什么名堂?”

阿基米德朝遥远的敌船一指说:“只要我们把罗马的战船消灭掉,他们就彻底失败了.而今天,他们灭亡的日子就要到啦,因为我们有太阳神助威.”阿基米德让士兵搬来几百面取火镜,在阿基米德的指挥下,一起往一艘战船的白帆上反射灼烈的阳光.不一会儿,被取火镜照射的白帆冒出缕缕青烟,海风一次,“呼”地起了火,火势一会儿就变大了.罗马侵略者狂叫起来,纷纷往海水里跳,有的烧死,有的淹死.后面的战船以为叙拉古人施了什么妖术,吓得调转船头便逃.

叙拉古国王兴奋地问阿基米德:“你这取火镜怎么真能向太阳神取来火呢?”

阿基米德说:“这镜子是凹面的镜子,它反射出的阳光,能集中到一点,这叫做焦点.焦点的温度很高很高,碰到易燃物上就能点着火.不过,假如没有太阳的帮忙,我们是无法取胜的.”

射向凹面镜的平行光经凹面镜反射后会聚于一点,使这点处的温度非常高,易于燃烧的物体处于此点处,很快就被点燃.

我国古代早就有利用“阳遂”聚光取火的记载,“阳遂”就是铜制的凹面镜.我们现在使用的太阳灶就是利用这一原理制成的.

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同类题3

阅读下面短文,掌握其大意,然后从所给的A、B、C、D中四个选项,选出最佳选项。

    As a boy I was always small for my age. I was five years younger than one of my brothers and seven years younger than the other. 1, I often felt left out when their friends came over to 2. I was too 3for whatever they were doing and they didn't want me to listen to their conversations either. More often than not I found myself outside playing  4and feeling forgotten.

    I remember one Spring afternoon I felt especially 5as I sat in the yard behind our house. We lived miles from town and I 6saw my own friends outside of school. I 7my brothers laughing from 8the house and felt a single tear coming down my face. At that moment I saw a large brown dog 9me. He was wagging (摇摆) his tail 10. He greeted me like a long lost11, licking (舔) my hand and sitting beside me on the grass,12he didn't know me. I petted and13this four-legged angel (天使). He let me pour out all my troubles and14my deepest thoughts before he kissed me goodbye. I went back inside feeling happy,15that no matter what life might hold I was loved. I16that the dog was sent in that moment of17to remind me just how much I was loved.

    In truth, nothing brings us greater18than knowing we are loved. Knowing we are loved gives us the19to love others as well. It also helps us to be the people the world meant for us to be. Remember how much the world loves you and share your20with the world.