What can be used to measure the percentage of oxygen inside a chain locker?（）

A.A black ball that is hoisted when the ship anchors
B.A buoy attached to the anchor
C.A buoy attached to the scope of an anchor chain
D.A mark of the number of fathoms in an anchor chain

A.Oil Rig chain and Oil Field Stud Link chain
B.Flash-butt welded chain and Di-Lok chain
C.Flash-butt welded chain and Oil Rig chain
D.Oil Field Stud Link chain and Flash-butt welded chain