Walter: Although cigarette smoking is legal, it should be banned on all airline flights. Cigarette smoking in the confines of an aircraft exposes nonsmokers to harmful secondhand smoke that they cannot avoid. Which one of the following principles, if established, would justify the proposal put forth by Walter? ['People should be prohibited from engaging in an otherwise legal activity in those situations in which that activity would unavoidably expose others to harm.', 'A legal activity that has the potential for causing harm to others in certain situations should be modified in those situations to render it harmless.', "If an activity is legal in some situations in which a person's engaging in that activity could harm others, then that activity should be legal in all situations.", 'An activity should be banned only if most situations in which a person engages in that activity would inevitably expose others to harm.']