nd. 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 he 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 univ
erse 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.
12. 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
13. 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
14. The word "constituents" in line 7 is closest in meaning to
(A) relatives
(B) causes
(C)components
(D) targets
15. Why does the author mention "cosmic rays't' 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
(D) To explain the abundance of hydrogen in the universe
16. The word "vary" in line 10 is closest ill meaning to
(A) mean
(B) stretch
(C) change
(D) include
17. The creation of helium within stars
(A) cannot be measured
(B) produces energy
(C) produces hydrogen as a by-product
(D) causes helium to be much more abundant In old stars than In young star:
18. The word "calculated" in line 15 is closest in meaning to
(A) ignored
(B) converted
(C) increased
(D) determined
19. 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
20. The word "ceased" in line 26 is closest in meaning to
(A)extended
(B)performed
(C)taken hold
(D)stopped
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