单项选择题

This is （）the river has been polluted.

A. what
B. why
C. that
D. whether

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单项选择题The United States （）a western country. The Philippines （）in the Pacific Ocean.

A. is; are
B. are; is
C. is; is
D. are; are

单项选择题This machine, （）for many years, is still working perfectly.

A. after which I have looked
B. which I have looked after
C. that I have looked after
D. I have looked after