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The first indications a mariner will have of the approach of a warm front will be （）.
A:large cumulonimbus (thunderclouds) building up
B:high cirrus clouds gradually changing to cirrostratus and then to altostratus
C:fog caused by the warm air passing over the cooler water
D:low dark clouds accompanied by intermittent rain

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出自：海船船员证

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