题干

关于电动势下列说法正确的是(   )

A:电源电动势等于电源正负极之间的电势差

B:用电压表直接测量电源两极得到的电压数值,实际上总略小于电源电动势的准确值

C:电源电动势总等于内、外电路上的电压之和,所以它的数值与外电路的组成有关

D:电源电动势总等于电路中通过1C的正电荷时,电源提供的能量

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答案(点此获取答案解析)

B,D

同类题3

阅读下列短文,从每题所给的四个选项(A、B、C和D)中,选出最佳选项。

    We live in a sweet world. An average American kid consumes(消耗) more than 20 teaspoons of sugar per day, and adults eat 50% more sugar today than they did in the 1970s. We all know that too much sugar isn't good for us. Researchers at the University of Utah used mice to make a study to prove the toxic effects of eating more sugar. They found it could influence people's health seriously.

    During the 58-week-long study, mice were fed a diet containing 25% more sugar. This percentage is equal to a healthy human diet along with three cans of soda daily. The team found that these mice were twice as likely to die as mice without eating so much sugar. Though the mice did not show signs of obesity or high blood pressure, male mice produced 25% fewer offspring(后代) than the other mice.

The reason why mice were chosen for research by scientists is not far to seek. “Mice have the genetic(基因)structure similar to humans'. Most substances(物质) that are harmful to mice are also harmful to people; it's likely that those important physical problems that cause the rising death rate among mice can also influence people,” says the study author James Ruff of the University of Utah. Findings from this study reveal negative effects that are not as detectable as weight gain or heart problems. Sugar can contribute to long-term changes in the body that can affect development and even shorten lives.

    Different types of sugar have different effects on the human body. Some of the sugar we consume comes from foods we might expect—candy, soda, and cookies. But much of the sugar that we eat is hidden. Food companies add large amounts of sugar to items we may not classify as sweets. To help cut down extra sugar consumption, nutrition experts suggest looking at the ingredients on food packages. Cutting sugar out of the American diet altogether may be difficult, but making every effort to control our nation's desire for sugar will avoid a sweeter future.

同类题4

阅读下文,回答问题

爸爸教我读中国诗

程怡

    ①十个月的时候,我得了一场可怕的脑膜炎,到了一岁半还不会说话,父母非常担心。一天,爸爸看报,我坐在他的膝上,指着某一个标题中的“上”字,爸爸说:“上?”我对他表示满意,赶紧从他的膝上爬下来,拽着他走到书箱前,得意扬扬地指着书箱外“函上”的“上”字,表明我认识这个字,这件事对父母而言,真是“上上大吉”!他们不再担心我有智力障碍了。之后,爸爸开始教我读诗。

    ②爸爸常教我念两个人的诗:一个是杜甫,一个是陆游。

    ③依稀记得,孩提时的一个夏夜,我困极了,趴在爸爸的膝上,爸爸摇着大蒲扇,满天的星斗朦朦胧胧的。“僵卧孤村不自哀,尚思为国戍轮台……”突然,爸爸那江西乡音很重的诵读声使我睁开了眼睛,我不知道那奇特的吟啸中有什么,但我一下子记住了这首诗。

    ④上学前我已经会背那首《示儿》:“死去元知万事空,但悲不见九州同。王师北定中原日,家祭无忘告乃翁。”爸爸问我懂不懂最后那句,我很得意地嚷嚷说:“那意思就是烧香磕头的时候别忘了告诉你爸爸!”当时,爸爸高兴得眼泪都流出来了。

    ⑤1959年秋,我上小学。那年冬天,爸妈因故很长时间不能住在家。姐姐是长女,照顾我和弟弟。一天晚上,爸爸出乎意料地出现在我们面前,令我们欢天喜地,难以入眠。躺在床上跟爸爸念杜甫的诗:“遥怜小儿女,未解忆长安。”爸爸问我懂不懂这诗句,我说:“我懂的,不过,爸爸想念我们的时候,我们也想念爸爸的。”爸爸不再说话,只是听我继续背他教我的诗。

    ⑥爸爸生命的最后几年,完全卧床不起。每当德沃夏克的大提琴协奏曲悲鸣的旋律在蕉影婆娑的窗边响起的时候,爸爸就会喃喃吟诵杜甫的诗。他告诉我,那一刻让他想起了故乡老宅,想起了祖母和母亲。

    ⑦那时我已在大学教中国古代文学,我理解父亲:人生无非家国之情,杜甫、陆游,我父亲他们这一代的知识分子,对家国,都有一种深情。父亲吟诗的声音,永远留在了我心底。

    ⑧很多年后,我看见报上某篇文章引了一首绝句,感觉就像遇到了一个老熟人。我没有念过那首诗,但我熟悉那种风格。回来一查,果然是陆游的诗,“征车已驾晨窗白,残烛依然伴客愁。”我当时的感受真是难以名状。爸爸在我童年时便种在我生命里的东西,突然宣告了它的无可移易的存在!

(有删改)