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单项选择题

The word "it" (Line 7, Par

A．6) probably refers to ________.A. carbon dioxideB. nitrogenC. atmosphereD. coal

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单项选择题Why researchers put trees in extra carbon dioxide in the first study

A．To learn whether trees can still absorb carbon dioxide under extreme conditions.
B．To get more oxygen from these trees.
C．To evaluate the maximum carbon dioxide that trees can absorb.
D．To see the growth rate of these trees.

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.