单项选择题

A．Middle-aged and elderly people.
B．Young working mothers.
C．University students and teachers.
D．Children and adolescents.
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单项选择题
A．They pollute the atmospheric environment.B．They produce dangerous gases.C．They stop the sun’s heat reaching the earth.D．They cause global warming.
A．They pollute the atmospheric environment.
B．They produce dangerous gases.
C．They stop the sun’s heat reaching the earth.
D．They cause global warming.
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