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5 10 15 20	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 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 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.
13. 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 was formed (D) Why hydrogen is abundant 14. 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 15. The word "constituents" in line 7 is closest in meaning to (A) relatives (B) causes (C) components (A) targets 16. Why does the author mention "cosmic rays" in line 7 ? (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 (A) To explain the abundance of hydrogen in the universe 17. The word "vary" in line 10 is closest in meaning to (A) mean (B) stretch (C) change (A) include 18. The creation of helium within stars (A) cannot be measured (B) produces energy (C) produces hydrogen as a by-product (A) causes helium to be much more abundant in old stars than in young stars 19. The word "calculated" in line 15 is closest in meaning to (A) ignored (B) converted (C) increased (A) determined 20. Most of th 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 (A) before most of the hydrogen 21. The word "ceased" in line 26 is closest in meaning to (A) extended (B) performed (C) taken hold (A) stopped
13. C 14. A 15. C 16. A 17. C 18. B 19. A 20. B 21. A