题目

关于DNA和RNA的叙述，正确的是（ ） A . DNA有氢键，RNA没有氢键 B . 病毒只含有DNA或者RNA一种核酸 C . 具有细胞结构的生物遗传物质主要是DNA D . 叶绿体、线粒体和核糖体都含有DNA 答案：B阅读下面材料，在空白处填入适当的内容或括号内单词的正确形式。Once upon a time there was a farmer1. worked hard in the fields every day. One day, while he was ploughing the fields, he heard a 2. ( bump) sound. He saw a 3. lying by the trump. He picked it4. and found that it was a fat rabbit. He thought to5. , “Since it is so easy to get a rabbit like that, why should I work so hard all day long?” He threw his hoe away and 6. (lie) by the tree every day, 7. (dream) for more rabbits to come. When he finally realized his8. (foolish) and returned to his fields, he found that all his crops were 9. (die). This story tells that we should not wait for unexpected gains by trusting to chance and luck. Or, we should not hope to get rewards10. hard work. .