单项选择题

We see lightening (before we hear) the thunder it (causes) because sound travels (more slowly) than (light is).

A. before we hear
B. causes
C. more slowly
D. light is

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单项选择题

I must work hard, however I'll fail in the exam.

A. must
B. hard
C. however
D. in

单项选择题

If the experiment succeeds or fails, it will provide us with valuable experience which is essential to improving our future work.

A. If
B. provide us
C. experience
D. improving

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