单项选择题 During a period of whiteout，you should expect which of the following（）.

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 （）.