钠用在电光源上做高压灯是因为（　　）

【甲】

    臣本布衣，躬耕于南阳，苟全性命于乱世，不求闻达于诸侯。先帝不以臣卑鄙，猥自枉屈，三顾臣于草庐之中，咨臣以当世之事，由是感激，遂许先帝以驱驰。后值倾覆，受任于败军之际，奉命于危难之间，尔来二十有一年矣。

    先帝知臣谨慎，故临崩寄臣以大事也。受命以来，夙夜忧叹，恐托付不效，以伤先帝之明，故五月渡泸，深入不毛。今南方已定，兵甲已足，当奖率三军，北定中原，庶竭驽钝，攘除奸凶，兴复汉室，还于旧都。此臣所以报先帝，而忠陛下之职份也。至于斟酌损益，进尽忠言，则攸之、祎、允之任也。

（节选自诸葛亮《出师表》）

【乙】

    诸葛亮围祁山，南安、天水、安定三郡反应亮。帝遣真督诸军军郿，遣张郃击亮将马谡，大破之。安定民杨条等略吏民保月支城，真进军围之。条谓其众曰：“大将军自来，吾原早降耳。”遂自缚出。三郡皆平。真以亮惩于祁山，后出必从陈仓，乃使将军郝昭、王生守陈仓，治其城。明年春，亮果围陈仓，已有备而不能克。增邑，并前二千九百户。四年，朝洛阳迁大司马赐剑履上殿入朝不趋。真以“蜀连出侵边境，宜遂伐之，数道并入，可大克也。”帝从其计。真当发西讨，帝亲临送。真以八月发长安，从子午道南入。司马宣王溯汉水，当会南郑。诸军或从斜谷道，或从武威入。会大霖雨三十余日，或栈道断绝，诏真还军。真每征行，与将士同劳苦，军赏不足，辄以家财班赐，士卒皆愿为用。真病还洛阳，帝自幸其第省疾。真薨，谥曰元侯。

【注】：①真，指曹真，三国时魏国名将。

（节选自《三国志·魏书》）

Dear Tony,

    I am very glad to get____ letter again. But I'm sorry to know that you have ____many problems. I really worry about you. Today I like to give you some____. I hope it can help you a little.

    You should be____outing and friendly than before. You should take part in more after-class____ with your classmates and make friends ____them. As for your studies, you should listen to your teachers____in class and if you have any questions, you should ____them for help. You said that you didn't understand what your English teacher said in class. You can tell____the problem. She can help you. Of course you can also get help from your classmates. I'm ____they will be kind to you. I wish everything will be OK.

    Huang Hua