2. 外语
  3. A summary of the physical and chemical nature of life must begin, not on the Earth, but in the Sun; in fact, at the Sun’s very

A summary of the physical and chemical nature of life must begin, not on the Earth, but in the Sun; in fact, at the Sun’s very

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问题     A summary of the physical and chemical nature of life must begin,
not on the Earth, but in the Sun; in fact, at the Sun’s very center. It is
here where is to be found the source of the energy that the Sun constantly【S1】________
pours out into the space as light and heat. This energy is liberated at the【S2】________
center of the Sun as billions on billions of nuclei of hydrogen atoms【S3】________
collide with each other and fuse together to form nuclei of helium, and,
in doing so, releases some of the energy that is stored in the nuclei of【S4】________
atoms.  The output of light and heat of the Sun requires that some 600
million tons of hydrogen are converted into helium in the Sun every【S5】________
second. This the Sun has been done for several thousands of millions of【S6】________
years.
    The nuclear energy is released at the Sun’s center as high-energy
gamma radiation, a form of electromagnetic radiation like light and radio
waves, only of very much short wavelength. This gamma radiation is【S7】________
absorbed by  atoms  inside the Sun, to  be  reemitted at  slight longer【S8】________
wavelengths. This radiation, in its turn, is absorbed and remitted. As
the energy filters through the layers of the solar interior, it passes
through the X-ray part of the spectrum, eventually becoming light.  At
this stage, it has reached that we call the solar surface, and can escape【S9】________
into space, without being absorbed further by solar atoms.  A very small
fraction of the Sun’s light and heat emitted in such directions that, after【S10】________
passing unhindered through interplanetary space, it hits the Earth.
【S3】
选项
答案on—upon
解析 介词误用。一般情况下on和upon可以互换，但在一些约定俗成的固定结构中不可以互换，billions upon billions是固定搭配，表示“数十亿计的”，类似的还有thousands upon thousands、millions upon millions。
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