Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions.

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 25 percent helium by weight and somewhat less 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 significant 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 temperature were too high and particles of matter 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 so sufficiently that neutrons and protons could stick together. But the nuclear reactions that led to the formations of helium went on for only relatively short time. By the time the universe was a few minutes old, helium production had effectively ceased.

What does the passage mainly explain?

• A. How stars produce energy
• B. Why hydrogen is abundant
• C. When most of the helium in the universe was formed
• D. The difference between helium and hydrogen

• Câu 2: Mã câu hỏi: 36588

  According to the passage, helium is ________.

  • A. difficult to detect
  • B. the second-most abundant element in the universe
  • C. the most prevalent element in quasars
  • D. the oldest element in the universe

• Câu 3: Mã câu hỏi: 36589

  The word "constituents" is closest in meaning to _____.

  • A. components
  • B. causes
  • C. targets
  • D. relatives

• Câu 4: Mã câu hỏi: 36590

  Why does the author mention "cosmic rays"?

  • A. To explain the abundance of hydrogen in the universe
  • B. As an example of an unsolved astronomical puzzle
  • C. As part of a list of things containing helium
  • D. To explain how the universe began

• Câu 5: Mã câu hỏi: 36591

  The word "they" refers to _____.

  • A. radiation
  • B. cosmic rays 
  • C. particles
  • D. constituents

• Câu 6: Mã câu hỏi: 36592

  The word "vary" is closest in meaning to __________.

  • A. stretch
  • B. include
  • C. mean
  • D. change

• Câu 7: Mã câu hỏi: 36593

  The creation of helium within stars ______.

  • A. produces energy
  • B. cannot be measured
  • C. causes helium to be much more abundant in old stars than in young stars
  • D. produces hydrogen as a by-product

• Câu 8: Mã câu hỏi: 36594

  Most of the helium in the universe was formed ______.

  • A. during the first minute of the universe's existence
  • B. in interstellar space
  • C. in a very short time
  • D. before most of the hydrogen