Suppose Someone Objected to McMahan's Criterion of Responsibility as Follows

Suppose someone objected to McMahan's criterion of responsibility as follows: The criterion of responsibility entails that even just combatants may not kill enemy combatants who are in no way morally responsible for their participation in the war. But the right to self-defense gives just combatants the right to protect their own lives against enemy attacks, and sometimes killing enemy combatants is the only way to do that. Therefore, the criterion of responsibility fails. Based on what he says in "The Ethics of Killing in War," how do you think McMahan would respond to this objection?

A) If unjust combatants are actively attacking just combatants, then even if they were forced to participate in the war, they have made themselves liable to defensive force. Therefore, the criterion of responsibility does not entail that just combatants may not kill them.
B) The right to self-defense gives everyone the right to defend their own lives against any attack. Furthermore, the right to self-defense overrides the obligations established by the criterion of responsibility. Therefore, just combatants do have the right to kill enemy combatants who bear no moral responsibility for their participation in the war.
C) If they are genuinely not morally responsible for their participation in the war, then they are morally equivalent to innocent bystanders, and even just combatants are prohibited from killing them. The criterion of responsibility gets the right answer in this case.
D) Although the ethics of killing in war are often hard to apply, the laws of war must be clear. Thus, although it might be morally wrong for just combatants to kill enemy soldiers in this case, it is and should remain legally permissible for them to do so.