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单项选择题

(21) summary of the physical and chemical (22) of life must begin, not on the Earth, but (23) the Sun; in fact, (24) the Sun’s very center. (25) is here that is to be found the (26) of the energy that the Sun constantly (27) space (28) light and heat. This energy is liberated at the center of the Sun as billions upon billions of nuclei hydrogen atoms (29) each other and (30) to form nuclei of helium, and, (31) doing so, (32) some of the energy that is stored in the nuclei of atoms. The output of light and heat of the Sun (33) that some 600 million tons of hydrogen (34) helium in the Sun every second. This the Sun (35) for several thousands of millions of years.

（24）（）

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B.on
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D.to

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