单项选择题

At the end of the last paragraph, the word "it" refers to ______.

A．water
B．the liquid sample
C．a closed loop
D．an electric spark
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单项选择题
Harold Urey and Stanley L. Miller did their experiment in order to ______.
A．dissolve some new chemicals
B．find out what may have happened on the Earth 4 billion years ago
C．simulate bolts of lightning on the ancient Earth
D．analyze a liquid
单项选择题
When did scientists come to realize how the atoms and molecules on the Earth mostlikely combined to make living things
A．4 billion years ago.
B．After seven days.
C．Many years ago.
D．In 1953.
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