Buoys which mark isolated dangers are painted with alternating（）.

A.black
B.yellow
C.green
D.red

A.A safe-water buoy will display red and white vertical stripes of reflective material
B.All reflective material is white because it is the most visible at night
C.A special-purpose mark will display either red or green reflective material to agree with its shape
D.A preferred-channel buoy displays either red or green reflective material to agree with the top band of color

A.a hard sand shoal marked with a light
B.completely submerged at all stages of the tide
C.surrounded by rocks awash at low water spring tides
D.a small，low，sandy islet surrounded by shoal water