2017年下半年公共英語四級專題模擬訓練

　　不知則問，不能則學，雖能不讓，然后為德。聞之不見，雖博必謬;見之而不知，雖識不妄;知之而不行，雖敦必困。以下是小編為大家搜索整理的2017年下半年公共英語四級專題模擬訓練，希望對正在關注的您有所幫助!更多精彩內容請及時關注我們應屆畢業(yè)生考試網(wǎng)!

　　Reading Comprehension

　　part A

　　Read the following two texts. Answer the questions below each text by choosing A.B.C or D. When was helium in the University formed?

　　The elements other than hydrogen and helium exist in such small quantities that it is accurate to say that the universe is somewhat more than 75 percent hydrogen.

　　Astronomers have measured the abundance of helium throughout our galaxy and in other galaxies as well. Helium has been found in old stars, in relatively young ones, in interstellar gas, and in the distant objects known as quasars. Helium nuclei have also been found to be constituents of cosmic rays that fall on the earth (cosmic “rays” are not really a form of radiation; they consist of rapidly moving particles of numerous different kinds). It doesn’t seem to make very much difference where the helium is found. Its relative abundance never seems to vary much. In some places, there may be slightly more of it; in others, slightly less, but the ratio of helium to hydrogen nuclei always remains about the same.

　　Helium is created in stars. In fact, nuclear reactions that convert hydrogen to helium are responsible for most of the energy that stars produce. However, the amount of helium that could have been produced in this manner can be calculated, and it turns out to be no more than a few percent. The universe has not existed long enough for this figure to be significantly greater. Consequently, if the universe is somewhat more than 25 percent helium now, then it must have been about 25 percent helium at a time near the beginning.

　　However, when the universe was less than one minute old, no helium could have existed. Calculations indicate that before this time temperatures were too high and particles of mater were moving around much too rapidly. It was only after the one-minute point that helium could exist. By this time, the universe had cooled sufficiently that neutrons and protons could stick together. But the nuclear reactions that led to the formation of helium went on for only a relatively short time. By the time the universe was a few minutes old, helium production had effectively ceased.

　　1.What does the passage mainly explain?

　　A.How stars produce energy.

　　B.The difference between helium and hydrogen.

　　C.When most of the helium in the universe hydrogen.

　　D.Why hydrogen is abundant.

　　2.According to the passage, helium is ____.

　　A.the second-most abundant element in the universe

　　B.difficult to detect

　　C.the oldest element in the universe

　　D.the most prevalent element in quasars

　　3.Why does the author mention “cosmic rays” in paragraph 2?

　　A.as part of a list of things containing helium.

　　B.As an example of an unsolved astronomical puzzle.

　　C.To explain how the universe began.

　　D.To explain the abundance of hydrogen in the universe.

　　4.The creation of helium within stars ____.

　　A.cannot be measured

　　B.produces energy

　　C.produces helium to be much more abundant in old stars than in young stars.

　　5.Most of the helium in the universe was formed _____.

　　A.in interstellar space

　　B.in a very short time

　　C.during the first minute of the universe’s existence

　　D.before most of the hydrogen

　　解析：

　　1.C

　　本文的題目是When was helium in the University formed?根據(jù)題目并結合文章的內容，我們可以判斷，本文主要介紹的是氦元素是何時在宇宙中形成的。

　　2.A

　　在第一段中我們知道：宇宙中除了氫和氦的數(shù)量比較多之外，其它元素的數(shù)量都很少。而且，氫在宇宙元素總量中占75%，所以氦元素在宇宙中一定是占第二位，僅次于氦。

　　3.A

　　文章第二段中介紹了許多包含氦的物質，包括in old stars, in relatively young ones, in interstellar gas, and in the distant objects known as quasars同時作者還提到了宇宙射線，并告訴我們宇宙射線中也包含氦。因此，作者在這里也提到了cosmic rays的'目的是要使人們知道另外一種包含氦的物質，具體而言，就是說宇宙射線也是眾多包含氦元素的物質中的一部分。

　　4.B

　　第三段開頭有這樣一句話：In fact, nuclear reactions that convert hydrogen to helium are responsible for most of the energy that stars produce. 這句話的意思是，星體產(chǎn)生的多數(shù)能量來源與巴氫轉變?yōu)楹さ脑�子核反應。也就是說，星體上的氫轉變?yōu)楹さ脑�子核反應產(chǎn)生了能量，這與選項A中的“星體中的氦的創(chuàng)造過程中產(chǎn)生了能量”是一致的。

　　5.B

　　文章的末尾有這樣一句話：But the nuclear reactions that led to the formation of helium went on for only a relatively short time. 這句話的意思是，但是由于導致氦形成的原子核反應持續(xù)的時間相對較短。文章在最后指出，在宇宙形成的早期，氦并不存在，因為那時的溫度太高。后來隨著溫度逐漸降低，中子和質子才得以粘在一起。原子核反應形成了氦，但這一過程進行的時間很短。到了the universe was a few minutes old的時候，氦的生產(chǎn)就停止了。

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