题干

人体进行各项生命活动所需的能量来自(  )

A:消化作用

B:吸收作用

C:呼吸作用

D:光合作用

上一题 下一题 0.0难度 选择题 更新时间:2018-05-22 09:14:53

答案(点此获取答案解析)

C

同类题1

完形填空
       Once upon a time there was a little orphan(孤儿) who always wished that he could fly like a bird. It was very   1   for him to understand why he could not fly.
  There was another little boy who was crippled(瘸子). One day the orphan came to a park, where he saw the little crippled boy   2   in the sand alone.
  He ran over to the little crippled boy and asked him if he had ever wanted to fly like a bird.
  “No, ” he said. “  3  I've been thinking about what it would be like to walk and run like   4  .”
  “That is very sad, ” said the little orphan. “Do you think we could be friends?”
  “Sure, ”said the little crippled boy.
  The two little boys played for hours.  5 of them felt as lonely as they have been before. Then the little crippled   6 father came with a wheelchair to  7  his son. The little orphan ran over to him and said something into his ear.
  “That would be Okay, ”said the man.
  The little orphan ran back to his friend and said, "You are my only friend and I   8 I could do something to make you walk and run like others. 9  , I can't. But there is something that I can do for you."
  The little orphan boy turned10  and told his friend to slide onto his back. Then he began to run across the grass. He ran faster and faster,  11 the little crippled boy on his back.
The father began to cry with tears as his son shouted at the top of his   12 , "I am FLYING! Daddy, I am FLYING!"
That's the little orphan who helped the crippled boy  13 the dream to fly in his life and moved the father.14 he didn't know the reason why he could not fly, he helped another boy fly and harvested happiness from it. Let's always be ready to help others because helping others is helping   15 .

同类题5

阅读《远距离输电》.

远距离输电

为了合理地利用自然资源,水电站要建在水库大坝上,以煤为燃料的火电站最好建在煤矿附近.但是用电的地方可能距离发电站很远,因此为了使千家万户都能用点,就要借助如图1所示的那些跨山越水的巍峨铁塔上的电缆来输送电能.

家庭用电的电压是220V,然而高压输电线上的电压通常有几百千伏.为什么要用这么高的电压输送电能呢?这是因为采用高压输电,可以减少由于电流热效应导致的电能在输送线路上的损耗.我们知道,输电线上损耗的功率P=I2R线,而输电导线的电阻R线很难大幅度减小,所以只能减小输电电流I.发电站的输出功率等于输出电压和输出电流的乘积,即P=UI.如果发电站输出功率不变,若要减小输出电流,就必须升高输出电压.

远距离高压输电的电压至少需要几十万伏,而大型发电机的电压一般为10~20kV,不符合远距离输电的要求.怎样将发电机产生的电压升高呢?根据电磁感应原理制造的变压器解决了这一难题.图2为某远距离输电的示意图,升先用升压变压器将发电厂发出的交流电压升高到220kV后再送到交流电网上,远距离传输后经一次高压变电站将电压降至110kV,再输送到二次高压变电站将电压降至10kV,然后送到工厂、科研和生活区等目的地,再用低压变压器降至220V供家庭用户使用.

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