单项选择题


A．subordinate
B．conscious
C．permanent
D．spiritual
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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
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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