Answer to Question #321538 in Law for goddess

Question

X1, X2 and X3 are crew on a whaling boat. A and B are fanatical members of an organisation that campaigns for putting an end to whaling. A and B continuously ram their boat into the whaling boat in an effort to damage it. X1 fears that the actions of A and B might gouge a hole in the hull of the whaling boat, causing it to sink. In an effort to stop them, X1 blasts A and B from their boat with his boat’s water cannon. A and B fall into the water. A drowns. In the meantime, B gets hold of a lifebuoy, and hangs on to it.. X2 and X3 swim up to the lifebuoy, and try to hold on to it. The three men start sinking. B and X2 both hit X3, who cannot swim well and is very overweight, until he releases his hold on the lifebuoy and drowns. X1, X2 and B are saved

State which ground of justification B may rely on and briefly consider whether he can succeed. (Do not give the definition of the defence and do not discuss all the requirements of it. Confine your answer to discussing the relevant requirement of the defence.)