The ladder shall ______．

A. be secured in a position so that it rests firmly against the ship's side from which the discharge operations are carried out
B. have at least two replacement steps which are secured in position by a method different from that used in the original construction
C. have battens made of hardwood，or other material of equivalent properties，in one piece and not less than
D. have steps made of softwood，or other material of equivalent properties，made in one piece free of knots，having an efficient slip surface