单项选择题
A.Echoes from a buoy
B.Own ship's marker
C.Echo from a ship on the same course at the same speed
D.Echo from a ship on a reciprocal course at the same speed
单项选择题 Your radar displays your ship off center．As you proceed on your course， your ship’s marker moves on the PPI scope while echoes from land masses remain stationary.What is this display called（）.
单项选择题 Your ARPA has two guard zones.What is the purpose of the inner guard zone（）.
单项选择题 Your ARPA has automatic speed inputs from the log．Due to currents，the log is indicating a faster speed than the speed over the ground.What should you expect under these circumstances（）.