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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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

Scientists figured out that human bodies are basically made of（）

A．hydrogen, carbon, nitrogen, and oxygen atoms
B．water vapor, ammonia, carbon dioxide, methane and hydrogen
C．biomolecules and water
D．evolutionary molecules