单项选择题 As the propeller turns，voids are formed on the trailing and leading edges of the propeller blades causing a loss of propulsive efficiency，pitting of the blades，and vibration. These voids are known as（）.

单项选择题 As oil pipe line connections were broken，（）quantity of oil was caused to spill onto the wharf and into the dock water.

单项选择题 Anchors are prevented from running out when secured by the（）.