A.SetB.FrequencyC.PeriodD.FunctionThe time required for……

单项选择题The recording fathometer produces a graphic record of the（）.

A.bottom contour only up to depths of 100 fathoms
B.depth underneath the keel against a time base
C.contour of the bottom against a distance base
D.depth of water against a distance base

单项选择题The points on the earth’s surface where the magnetic dip is 90° are（）.

A.Along the magnetic equator
B.Connected by the isoclinal line
C.The isopors
D.The magnetic poles

单项选择题The four standard light colors used for lighted aids to navigation are red，green，white，and （）.

A.purple
B.orange
C.blue
D.yellow

单项选择题The equator is（）.

A.the primary great circle of the Earth perpendicular to the axis
B.the line to which all celestial observations are reduced
C.the line from which a celestial body's altitude is measured
D.All of the above

单项选择题Restricted areas at locks and dams are indicated by（）.

A.Flashing red lights upstream and fixed red lights downstream
B.Yellow unlighted buoys
C.Signs and/or flashing red lights
D.Red daymarks upstream and green daymarks downstream