单项选择题

_____ such heavy pollution already, it may now be too late to clean up the river.

A．Having suffered
B．Suffering
C．To suffer
D．Suffered
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单项选择题
Business letters must always be _____, but we should write in a natural way to friends.
A． formal
B．normal
C．former
D．casual
单项选择题
Only when we live in _______ with nature can we make the most out of it.
A．cooperation
B．interaction
C．integration
D．harmony
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