Shipboard Emergency Drills must be carried out at least（）on board cargo ship.

When the wave period and the apparent rolling period are the same（）.

The force resulting from the earth’s rotation that causes winds to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere is called（）.

Bales close to the deckhead should be covered to prevent（）.

Despite the rising importance and great convenience of the gyrocompass,the magnetic compass still retain its importance because of its（）.