单项选择题

The word "it" (Line 3, Par

A．6) probably refers to ______.A. carbon dioxideB. nitrogenC. atmosphereD. coal
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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 dioxid
单项选择题
What is scientists' attitude towards the future of Chad Lake
A．Pessimism.
B．Hopeful.
C．Indifferent.
D．Doubtful.
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