单项选择题
A.being raised too high B.swinging at sea C.overloading when not in use D.being lowered too low
A.mean high water B.mean low water C.half-tide level D.mean sea level
A.give better range performance in rain，hail，etc B.display small targets in a mass of dense sea clutter at a greater range C.have less sea return in choppy rough seas D.display a more maplike presentation for inshore navigation