Experts are trying to work out a solution to save the miners _____ its too late.

A. before
B. since
C. unless
D. when
A
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科目：高中英語 來源：四川省模擬題 題型：單選題

Experts are trying to work out a solution to save the miners _____ its too late.
[     ]
A. before
B. since
C. unless
D. when

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