单项选择题

A．Because they pollute air.
B．Because they are natural hazards.
C．Because they are increasing in numbers.
D．Because people don’’t walk so often.
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单项选择题
A．It is difficult to identify.B．It has been misplaced.C．It is missing.D．It has been borrowed by someone.
A．It is difficult to identify.
B．It has been misplaced.
C．It is missing.
D．It has been borrowed by someone.
单项选择题
A．The lifecycle of trees.B．The number of trees.C．The intensity of solar burning.D．The quality of air.
A．The lifecycle of trees.
B．The number of trees.
C．The intensity of solar burning.
D．The quality of air.
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