Vessel required to have an Automatic Radar Plotting Aid must have a device to indicate the（）.

单项选择题To determine if risk of collision exists，a vessel which is fitted with radar must use（）.

A.radar scanning
B.radar plotting
C.compass bearings
D.All of the above

单项选择题The shoreline along Rocky Point should give a good radar return because（）.

A.the shore is bluff and rocky
B.of offshore exposed rocks
C.submerged reefs cause prominent breakers
D.the lookout tower is marked with radar reflectors

单项选择题The radio waves used for radar are very short，only（）long.

A.a few centimeters
B.a few fathoms
C.a few meters
D.a few feet

单项选择题The radar control used to reduce sea return at close ranges is the（）.

A.gain control
B.sensitivity time control
C.fast time constant
D.pulse length control

单项选择题The radar control that shortens all echoes on the display and reduces clutter caused by rain or snow is the（）.

A.sensitivity time control (sea clutter control)
B.receiver gain control
C.brilliance control
D.fast time constant (differentiator)