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.
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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
What is the purpose of this passage
A．Introduce some new ideas about the relationship between trees and carbon dioxide.
B．Introduce recent condition of global industrial pollution.
C．Call on people to plant more trees to reduce greenhouse gases.
D．Point out that power companies should be responsible for the rising levels of carbon dioxide.
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