问题 单项选择题

______,在各种理化因素或环境因素的影响下,DNA 碱基排列顺序即遗传密码可能发生改变,从而引起基因突变。即使在正常情况下,DNA复制过程中也发生频率极低的突变。
文中画线处应该填入的是( )。

A.DNA的分子结构并不是一成不变的

B.DNA的分子结构发生变异的可能性很小

C.DNA的分子结构极易产生基因突变

D.DNA的分子结构只有在特殊情况才可能变异

答案

参考答案:A

解析:根据文中的关键字词“可能”、“即使在正常情况下”、“也发生”可知正确答案为A。

阅读理解

Aunt Polly had told Tom to paint the whole fence (栅栏)in front of the house. He thought of the other boys out for the day enjoying themselves and he felt very unhappy. Then he had an idea! He picked up the brush and started painting slowly and carefully. Soon, his friend Ben Rogers appeared, eating an apple. Tom pretended (假装) not to see him.

“Tom!” Ben said. “Why are you working?”

“Oh, it’s you, Ben,”Tom said, “I didn’t notice you.”

“I’m gong for a swim,” Ben said, “but you don’t’ want to come. You want to work, don’t you? It’s much nicer!” Ben laughed loudly. Tom looked seriously at Ben and said. “What is work? What do you call work?”

“Isn’t that work?” Ben asked.

“Well, no. Does a boy get the opportunity to paint a fence every day? You’ve never painted a fence, have you? Auntie says I have to do it with great care!’

“Pleas, Tom. Let me do a little,” said Ben.

“No, Ben. If you do, you’ll do it wrong. You can’t paint very well. If it was the back fence wouldn’t mind, but it isn’t.”

“I won’t do it wrong. I’ll be really careful. I’ll give you my apple.” Tom gave him the brush and paint with a sad face, but in his heart he was glad, His trick had worked.

After Ben got tired and left, other boys came along and Tom said the same things to them. Soon, the whole fence was painted white, but not by Tom. He felt very clever because he had discovered something important about the way people are. People only want what they can’t have or want to do what they shouldn’t do.

小题1:Which of the following has the closest meaning to the underlined word “opportunity” on this passage?

A.Reward

B.Interest.

C.Abilityy.

D.Chance.小题2:Why did Tom pretend not to see Ben?

A.Because he wanted Ben to believe that the was enjoying the work.

B.Because he would like to try his best to attract Ben’s attention.

C.Because he did not want Ben to see the fact that he had to work.

D.Because he was afraid that Ben might laugh at him if the saw him.小题3:What can we infer (推断) from the story?

A.Aunt Polly gave Tom a lot of pocket money for what he had done.

B.Ben and the other boys might feel unhappy when they found out the troth.

C.Tom believed that he had discovered how to deal with different people.

D.Everyone thought Tom clever when they realized what had happened.

单项选择题 A1型题