单项选择题
A strong back refers to a（）.
A.bar securing a cargo port B.centerline vertical bulkhead C.deep beam D.spanner stay
单项选择题 A spreader bar is used to（）.
单项选择题 A spark arrestor（）.
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（）.
