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  3. For many years, scientists couldn’t figure out how atoms and molecules(分子)on the Earth combined to make living things. Plants, f

For many years, scientists couldn’t figure out how atoms and molecules(分子)on the Earth combined to make living things. Plants, f

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问题     For many years, scientists couldn’t figure out how atoms and molecules(分子)on the Earth combined to make living things. Plants, fishes, dinosaurs(恐龙), and people are made of atoms and molecules, but they are put together in a more complicated way than the molecules in the primitive ocean. What’s more, living things, that have energy and can reproduce on the Earth four billion years ago were lifeless.
    After years of study, scientists figured out that living things, including human bodies, are basically made of amino acids and nucleotide(氨基酸与核苷酸)bases. These are molecules with millions of hydrogen(氢), carbon(碳), nitrogen(氮), and oxygen atom. How could such complicated molecules have been formed in the primitive soup? Scientists were stumped(难住).
    Then, in 1953, two scientists named Harold Urey and Stanley L. Miller did a very simple experiment to find out what had happened on the primitive Earth. They set up some tubes and bottles in a closed loop(线圈), and put in some of the same gases that were present in the atmosphere four billion years ago: water vapor, carbon dioxide(二氧化碳), methane(甲烷), and hydrogen.
    Then they shot an electric spark through the gases to simulate(模拟)bolts(闪电)of lightning on the ancient Earth, circulated the gases through some water, sent them back for more sparks, and so on. After seven days, the water that the gases had been bubbling(起跑)through had turned brown. Some new chemicals were dissolved in it . When Miller and Urey analyzed the liquid, they found that it contained amino acids—the very kind of molecules found in all living things.
At the end of the last paragraph, the word "it" refers to______.
选项 A、a close loop
B、an electric spark
C、water
D、the liquid
答案D
解析 文章最后一句话when Miller and Urey analyzed the liquid，they found that it contained…，“当他们对液体进行分析时，发现里面含有……”中得知，it指的就是从句中的liquid。
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