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.
Why does the author mention "cosmic rays"?
A. To explain the abundance of hydrogen in the universe
B. To explain how the universe began
C. As an example of an unsolved astronomical puzzle
D. As part of a list of things containing helium
Kiến thức: Đọc hiểu
Giải thích:
Tại sao tác giả đề cập đến “các tia vũ trụ”?
A. Để giải thích sự phong phú của hydro trong vũ trụ B. Để giải thích về việc vũ trụ bắt đầu như thế nào
C. Để lấy ví dụ về câu đố về thiên văn chưa giải quyết D. Nó là một phần của danh sách những thứ có chứa khí heli
Thông tin: Helium nuclei have also been found to be constituents of cosmic rays that fall on the earth
Tạm dịch: Hạt nhân helium cũng đã được tìm thấy là thành phần của các tia vũ trụ chiếu xuống Trái đất
Chọn D