The extension of the after part of the keel in a single-screw vessel upon which the stern post rests is called the（）.

A.the reinforcement of the aft transverse watertight bulkhead
B.the double bottom structure in way of the foremost cargo hold
C.the introduction of a more rigorous survey regime and greater attention to operating procedures
D.introduction of new and improved designs

A.frames to which the tank top and bottom shell are fastened on a double bottomed ship
B.transverse members of the ships frame which support the decks
C.longitudinal beams in the extreme bottom of a ship from which the ship's ribs start
D.longitudinal angle bars fastened to a surface for strength