A drilling bit which is doughnut shaped to permit recovery of the center portion of the hole drilled is called a（）.

单项选择题A drawbridge may use visual signals to acknowledge a vessel’s request to open the draw. Which signal indicates that the draw will NOT be opened immediately?（）

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（）.

A.at all times
B.in the bottom return mode
C.in the volume reverberation mode
D.only when there is no current

单项选择题A disadvantage of using chain lashing on heavy vehicles aboard Ro-Ro vessels is that it is （）.

A.heavy
B.easily damaged
C.affected by temperature
D.costly relative to the strength ratio

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（）.

A.stopper
B.screen
C.filter
D.restrictor

单项选择题A decrease in barometric pressure is associated with all of the following except（）.

A.rising warm air
B.proximity to a low pressure area
C.inward spiraling circulation
D.clear dry weather