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On navigational aids，what does the light characteristic Fl(2+1) mean?（）
A:A flashing light combined with a fixed light of greater brightness
B:Light flashes combined in groups，with a different number of flashes in each group
C:A light showing groups of two or more flashes at regular intervals
D:A fixed light varied at regular intervals by groups of two or more flashes of greater brightness

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出自：海船船员证

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