单项选择题 A sea anchor is（）.

单项选择题 A mercurial barometer at sea is subject to rapid variations in height (“pumping”) due to the pitch and roll of the vessel. To avoid this error，measurements of atmospheric pressure at sea are usually measured with a(n)（）.

单项选择题 A heavy steel device that is set on the sea floor and used as a drilling template in offshore drilling operations is called a（）.