单项选择题
A.Snowfall or blowing snow
B.Lack of ability to estimate distance
C.Harsh contrast between sun-illuminated snow cover and the background
D.Hazy horizons with extensive mirage effects
单项选择题 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 （）.
单项选择题 Clearances of bridges and overhead cables below Bonneville Dam refer to heights in feet above mean （）.