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

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.

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.