单项选择题 A tank 36 ft. by 36 ft. by 6 ft. is filled with water to a depth of 5 ft. If a bulkhead is placed in the center of the tank running fore-and-aft along the 36-foot axis，how will the value of the moment of inertia of the free surface be affected?（）

单项选择题 A tackle is “two blocked” when the blocks are（）.

单项选择题 A swift current occurring in a narrow passage connecting two large bodies of water，which is produced by the continuously changing difference in height of tide at the two ends of the passage，is called a（）.