A tug is to assist in docking an oceangoing vessel on a hawser. The greatest danger to the tug is （）.

Conditions for crossing a rough bar are usually best at （）.

A pilot vessel on pilotage duty at night will show sidelights and a sternlight （）.

The meaning of ebb tide is that （）.

The distance between any two meridians measured along a parallel of latitude （）.

The Earth’s irregular heating is caused by （）.

The height of a light is measured from which reference plane?（）

Clearance gauges at bridges indicate （）.

During a period of whiteout，you should expect which of the following（）.

The period at high or low tide during which there is no change in the height of the water is called the （）.