单项选择题

The principal reason for the great number of smoke is that there are too many factories in this area.

A．principal
B．number
C．that
D．too many
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When Mr. Green retired his son ______ the business.
A．took up
B．took on
C．took over
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The news of the air crash is not yet ______ by the government.
A．committed
B．confirmed
C．considered
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