单项选择题

Small floes of rough，hummocky sea ice capable of damaging a vessel（）.

A.can usually be detected by radar in a smooth sea at a range of 4 to 6 kilometers
B.are indistinguishable from sea return on the PPI
C.are invisible to radar when covered with a thick layer of snow
D.are usually seen at night before they are close enough to provide a radar echo

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