单项选择题
A single fitting installed in a pipeline that either blanks off the pipe or allows a full flow passage of a liquid through the pipe is referred to as a（）.
A.blind flange B.pivot coupling C.quick-release coupling D.spectacle flange
单项选择题 A ship crane which has been idle for a period of over six months shall be inspected to the same standards as a（）.
单项选择题 A sheepshank is used to（）.
单项选择题 A sextant having an index error that is off the arc has a（）.
