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.

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.