单项选择题

According to the writer, living things on Earth have included ______.

A．atoms and molecules
B．chemicals
C．plants, fish, dinosaurs and human beings
D．the primitive soup
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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
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
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