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.

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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.

单项选择题

In a laughter clinic, doctors

A．laugh at their patients.
B．encourage their patients to laugh.
C．smile when they don't feel like laughing.
D．never stop laughing.