Echo sounder sends a（） (无线电信号)from the bottom of the ship to the seabed.

单项选择题Radar is of great help to ship in poor（）(能见度).

A.visibility
B.VHF
C.visible
D.compass

单项选择题Navigators use navigational aids, such as（）（罗经）.

A.sextant
B.binoculars
C.compass
D.loran

单项选择题（）（电罗经）can tell us true north.

A.VHF
B.Radar
C.Chart
D.Gyro-compass

单项选择题Officers observe ships with a pair of （）（望远镜）.

A.sextant
B.binoculars
C.compass
D.radar

单项选择题The （）（天线）is like a dish.

A.line
B.thread
C.aerial
D.wire