问题 不定项选择

雷达是运用电磁波来工作的,它发的电磁波频率多在300 MHz至1000 MHz的范围内,已知真空中光速 m/s,下列说法正确的是[ ]

A.电磁波可由恒定不变电场和磁场产生

B.电磁波可由周期性变化的电场或磁场产生

C.雷达发射的电磁波在真空中的波长范围多在0.3 m至1 m之间

D.雷达与目标之间的距离可由电磁波从发射至接收的时间间隔确定

答案

BCD

阅读理解

Gina was a 3rd grader. She was very clever, but she had no friends at school until Rita joined her class. New to the city and too shy to make any friends, Rita did not mind being Gina's friend. At least she had someone to play with.

One day the teacher scolded (训斥) Rita for not concentrating when she was doing some math problems. The whole class laughed at her and Rita cried hard. The teacher had patiently explained how to do them, but Rita was just not able to understand.

Gina told her mother about Rita's bad day. "Why don't you try to help her?" asked her mother. " She likes you very much."

" Mommy, I don't have time to waste." said Gina.

" But Rita is your friend and if you don't help her, who will?" said her mother. But Gina didn't think so.

" Then Gina's mother decided to act tough(强硬地)." If you don't help her, I will not let you watch cartoons for a week."

Extremely unhappy, Gina taught Rita. Rita understood, for Gina knew how to explain things simply. Rita was excited and told everyone what a good friend Gina was.

One day, Gina made a mistake, and the principal (校长) asked Gina to pick up the waste paper in every classroom after school.

After school, Gina began to clean the classrooms and she felt very sad. Rita found out about it and went to help her do it.

From that day, Gina and Rita became the best of friends.

小题1: Did Rita mind making friends with Gina?

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小题2: What happened after the teacher scolded Rita?

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小题3: Why did Rita understand the math problems after Gina taught her?

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小题4:How did Gina feel when she was made to pick up waste paper?

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小题5:What can we learn form the story?

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单项选择题

 原始地球形成后的八亿年,其内部逐渐变热使局部熔融并超过铁的熔点,其使地球中的金属铁、镍及硫化铁熔化,并因密度大而流向地球的中心部位,从而形成液态铁质地核。同时,地球的平均温度进一步上升(可达约2 000摄氏度),引起地球内部大部分物质熔融,比母质轻的熔融物质向上浮动,把热带到地表,经冷却后又向下沉没。这种对流作用控制下的物质移动,使原始地球产生全球性的分异,演化成分层的地球,即中心为铁质地核,表层为低熔点的较轻物质组成的最原始的陆核,陆核进一步增生,扩大形成地壳。地核与地壳之间为地幔。分异作用是地球内部最重要的作用,它导致了地壳及大陆的形成,并导致大气和海洋的形成。所以说,我们的地球是原始地球再生的,这个再生过程大约发生在40亿年前(或说37亿年前至45亿年前之间),即我们已经发现的最古老岩石的形成时期之前氢和氧合成的水,原先潜藏于一些矿物中。当原始地球变热并部分熔融时,水释放出来并随熔岩运移到地表,人部分以蒸气状态逸散,其余部分在漫长的地质历史进程中逐渐充满大洋。在原始地球变热而产生分异作用的过程中,从地球内部释放出来的气体形成了大气圈。早期地球的大气圈成分与现代不同,正是由于紫外辐射的能量促使原始大气成分之间发生反应,从无机物质生成有机小分子,然后发展成有机高分子物质组成的多分子体系,再演变成细胞,生命得以开始和进化。

对文章内容,分析有误的—项是()。

A.地球一直处于发展变化过程中,至今依然如此

B.地球形成的历史不超过50亿年

C.生命产生于大气成分之间的反应

D.氢和氧结合成的水最终大部分汇集成了海洋