单项选择题

This is an illness that can result in total blindness（）left untreated.

A. after
B. if
C. since
D. unless

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单项选择题

The physicist has made a discovery, （） of great importance to the progress of science and technology.

A. I think which is
B. that I think is
C. which I think is
D. which I think it is

单项选择题

Acute hearing helps most animals sense the approach of thunderstorms long before people（）

A. do
B. hear
C. do them
D. hearing it

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