单项选择题

()is a good form of exercise for both young and old.

A．Walking
B．To walk
C．The walk
D．Walk

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单项选择题

()of the river has been polluted.

A．Second-three
B．Two-three
C．Two-third
D．Two-thirds

单项选择题

Since his children have all grown up, he has nothing now.()

A．to worry about
B．worrying about
C．to be worried
D．worried

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