单项选择题

The word "it" (Line 7, Para. 6) probably refers to ________.

A．carbon dioxide
B．nitrogen
C．atmosphere
D．coal
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单项选择题
What happened to the leaves falling from the trees in the second study
A．They broke down and the main parts turned into oxygen.
B．They broke down and the carbon content had mainly turned into carbon dioxide.
C．They broke down and the carbon content was mainly absorbed in the soil.
D．They absorbed more carbon dioxide.
单项选择题
How many instruments are mentioned in the passage
A．Two.
B．Three.
C．Four.
D．Five.
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