问题：

[单选] 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