单项选择题 One of the first actions to be taken by survivors when they have boarded an inflatable liferaft is to（）.

单项选择题 Normal pool elevation is the height in feet of the section of river above a dam. This height is measured from（）.

单项选择题 Most drill ships and barges have a walled opening below the derrick，open to the water’s surface and through which various drilling tools can pass down to the sea floor called a （）.