单项选择题
A.A flashing amber light
B.A fixed red light
C.A white flag raised and lowered vertically
D.A flashing white light
单项选择题 A Doppler speed log indicates speed through the water（）.
单项选择题 A disadvantage of using chain lashing on heavy vehicles aboard Ro-Ro vessels is that it is （）.
单项选择题 A device fitted over the discharge opening on a relief valve consisting of one or two woven wire fabrics is called a flame（）.