DNA是由4种单核苷酸按特定顺序组成的，一个单核苷酸相当于一个密码，3个相邻的密码组成一个密码子，十几个至上千个密码子有序地连接起来，就形成一个遗传功能单位，人们称之为结构基因。
最准确理解上面一段话意思的是______。
A．4个按特定顺序组成的单核苷酸就是DNA
B．结构基因是由十几个至上千个密码子有序地连接起来形成的
C．密码子是由3个单核苷酸组成的
D．单核苷酸就是遗传功能单位，它由十几个至上千个密码子组成
参考答案：B
解析：这段文字的中心话题是DNA和结构基因。A项说法错误，根据文段第一句话，应该是“4种按特定顺序组成的单核苷酸就是DNA”，而不是4个，故可排除；C项不是文段的中心话题，故可排除；D项说法错误，根据“一个单核苷酸相当于一个密码，3个相邻的密码组成一个密码子，十几个至上千个密码子有序地连接起来，就形成一个遗传功能单位”，可知十几个至上千个密码子有序地连接起来，就形成一个遗传功能单位，而不是单核苷酸就是遗传功能单位。故选B。
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| Sarah came running in. "Look what l found." Over the top of the paper I was reading came a long object that caused me to 1 . It was a snake skin. "Isn't it 2 ?" said my wide-eyed seven-year-old. I stared at it thinking to myself that it really wasn't 3 beautiful, but I have learned never to appear uninterested 4 tired of children. " 5 does a snake shed its skin?" Sarah asked. I also try to seize every 6 to teach my children there is almost something beyond the obvious. "Snakes shed their skin because they 7 to renew themselves," I explained. As is so often the 8 in my family, the original subject leads to 9 and another, until we are discussing something quite 10 . "Why do they need to renew themselves?" Sarah asked. Robert replied humorously. "Because they don't like 11 they are and they want to be someone else." Sarah and I politely 12 her brother. "We often need to shed our skins," I said to my now 13 daughter. "This snake no longer needs this skin. Probably he doesn't think he looks as 14 in it as he once did. Like buying a new suit." Of course, I'm sure this explanation won't 15 naturalists (博物学家). But Sarah was understanding. As we 16 , I knew that she began to understand that renewal is part of 17 and see what we need to keep and what we need to 18 . I was careful to point out that this is a 19 process, not one to be forced. "Snakes don't peel off their skin when they feel like it," I explained. "It 20 as a natural result of their growth." "I see, Dad," said Sarah and jumped off my lap, grabbed the snake skin, and ran off. | ||||
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