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【S1】______
the Sun constantly pours out into the space as light and heat. This【S2】______
energy is liberated at the center of the Sun as billions on billions of【S3】______
nuclei of hydrogen atoms collide with each other and fuse together to
form nuclei of helium, and, in doing so, releases some of the energy【S4】______
that is stored in the nuclei of atoms. The output of light and heat of
the Sun requires that some 600 million tons of hydrogen are【S5】______
converted into helium in the Sun every second. This the Sun has
been done for several thousands of millions of year.【S6】______
    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【S7】______
gamma radiation is absorbed by atoms inside the Sun, to be
reemitted at slight longer wavelengths. This radiation, in its turn, is【S8】______
absorbed and remitted. As the energy filters through the layers of
the solar interior, it passes through the x-ray part of the spectrum,
and eventually becoming light. At this stage, it has reached that we【S9】______
call the solar surface, and can escape 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 passing【S10】______
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，millionsupon millions。
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