Due to lapse of time the risk in these areas to surface navigation is now considered （）than the ordinary risks of navigation，but a very real risk still exists with regard to anchoring，fishing or any form of submarine or seabed activity.

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 （）.

A.lower low water
B.high water
C.low water
D.sea level

单项选择题Clearance gauges at bridges indicate （）.

A.the height of the tide
B.depth of water under the bridge
C.charted vertical clearance at mean low water
D.distance from the water to low steel of the bridge

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．

A.concealed
B.buried
C.existed
D.built

单项选择题Attention is called to the fact that in many cases charted depths may be decreased by up to （）.

A.one nautical mile
B.two kilometers
C.5 cables only
D.one fathom