单项选择题
Conditions for crossing a rough bar are usually best at （）.
A.low water slack B.high water slack C.high water ebb D.high water flood
单项选择题 Clearances of bridges and overhead cables below Bonneville Dam refer to heights in feet above mean （）.
单项选择题 Clearance gauges at bridges indicate （）.
单项选择题 Attention is called to the fact that in many cases pipelines are not （） and charted depths may be decreased by up to 2 meters．
