单项选择题

A．Two years.
B．Three years.
C．Four years.
D．Five years.
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单项选择题
A．More and more cities appear.B．There are increasing complaints about environmental problems.C．More and more people pour into cities.D．One-third of industrial sewage in the cities went untreated.
A．More and more cities appear.
B．There are increasing complaints about environmental problems.
C．More and more people pour into cities.
D．One-third of industrial sewage in the cities went untreated.
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engineer
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