下列关于动脉血和静脉血，叙述正确的是（    ）

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    People in many countries are learning English.Some of them are small children. Others are teenagers. Many are adults. Some learn at school, others learn by themselves. A few people learn English by hearing the language over the radio, on TV or in films. As we know, it's hard to learn a foreign language well. One must work hard to learn another language. Because we can't use it often. But if you have right ways to learn English, it's a piece of cake.

    Why do all these people want to learn English?It is difficult to answer this question. Many boys and girls learn English at school because it is one of their subjects required (要求) for study. They study their own language, math and English. Some people learn it because it is useful for their work. Many people learn English for their higher studies,because at colleges or universities some of their books are in English. Some people learn English because they want to read newspapers or magazines in English.

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材料  中国第28次南极科学考察队凯旋。此次南极科学考察历时163天，圆满完成“一船三站”考察任务。“雪龙”号极地科学考察船安全完成天津港澳大利亚弗里曼特尔港中山站长城站阿根廷乌斯怀亚港中山站澳大利亚弗里曼特尔港上海的航行任务，总航程28000余海里，其中冰区航行3900余海里。