单项选择题

A．mistakes
B．misunderstanding
C．calculation
D．problems
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单项选择题
A．necessaryB．importantC．availableD．changeable
A．necessary
B．important
C．available
D．changeable
单项选择题
According to the passage, a greenhouse is built to
A．keep the plants warm.
B．prevent air pollution.
C．stop the greenhouse effect.
D．produce useful gases.
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