单项选择题

A major cause of the increase in the greenhouse gases is

A．the burning of trees and crops.
B．the building of many greenhouses.
C．the burning of large quantities of coal and oil.
D．the flooding of large areas of the world.

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单项选择题

According to the passage, chlorofluorocarbon gases differ from carbon dioxide in that

A．they are more useful.
B．they are much heavier.
C．they are more efficient in making the world colder.
D．they are more effective in catching the heat from the sun.

单项选择题

Without the greenhouse gases, the earth would

A．become warmer.
B．be much colder.
C．move more slowly.
D．move quickly.