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    29、If we know about what causes endangerment, we may be able to takes measures______ it is too late.

    A. until                  B. unless                      C. when                       D. before

    29、D

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    If we know about what causes endangerment, we may be able to takes measures______ it is too late.

    A. until                  B. unless                      C. when                       D. before

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