火箭在大气中飞行时，它的头部磨擦发热，温度可达几千摄氏度。在火箭上涂一层特殊材料，这种材料在高温下熔化并且汽化，能起到保护火箭头部的作用，这是因为（    ）

四川芦山地震后，许多健康公民无偿献血，医生给我们抽血时，针刺入的血管是（    ）

完形填空：阅读下面短文，从短文后各题所给的四个选项（A、B、C和D）中选出可以填入空白处的最佳选项。
    John entered a college and often worked late until 12 at night. He fell into the 1 of smoking a year ago. He smoked so much that he 2 had a cigarette on his lips. He 3 while he read, while he watched television and 4 while he drank a cup of coffee or tea. He smoked nearly forty cigarettes a day, but he was 5 about it.
    John's friend, Frank, 6 many times to 7 him not to smoke. “Smoking is bad for your 8!” he said, “You'd better give it up. ” But his advice didn't  9.
    Before long, John 10 that he often got a bad cough, 11 he went to see a doctor. The doctor 12 him carefully and asked whether he often smoked. John told him the 13. Then the doctor told him that his cough was 14 by smoking and he wouldn't get any better unless he stopped smoking.
“If not,” said the doctor, “you will never 15 from your cough and some other illnesses may also be caused.” John became 16 and made up his mind to give up smoking. At the beginning, it was very 17 for him. Every 18 when he wanted to smoke, he got something to eat. At last, he was not 19 smoking at all. And he no longer had 20.

对外交往是国家实力的展示。中国需要世界,世界也需要中国。阅读下列材料,回答问题。

【经济交流】

材料一  丝绸之路的路线图

材料二  郑和七次下西洋,海上丝绸之路盛极一时。他每到一地,都以中国的丝绸和瓷器换取当地的特产或馈赠当地的国王,与当地居民公平交易,互通有无,不占别国一寸土地,未掠夺他人一分财富。

——《郑和下西洋——伟大的和平使者》

【文化交流】

材料三

【政治交往】

材料四  中华人民共和国成立后,美国对新中国采取政治上孤立、经济上封锁、军事上包围的政策,中美关系长期以来处于敌对状态。20世纪70年代,随着国际形势的变化,中美两国领导人都认为有必要改善两国关系,实现双边关系正常化。

——中华书局《中国历史》(八年级下)   

19世纪中期，美国经过内战，扫除了资本主义发展的又一障碍，为以后的经济迅速发展创造了条件。这里所说的“又一障碍”是指（  ）