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

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

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