At the instant when a string of casing being run from a ship is landed in the well head,（）.
A.the total weight in air of the casing string is removed from the ship
B.the hook load (weight of the casing string) is removed from the ship
C.the weight of the casing inside the riser is removed from the ship
D.the weight of the casing string added to the weight of the fill-up mud is removed from the ship
