题目内容
(请给出正确答案)
[单选题]
On navigational aidswhat 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 groupswith 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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