Activities that pose risks to life are acceptable if and only if each person who bears the risks either gains some net benefit that cannot be had without such risks, or bears the risks voluntarily. Which one of the following judgments most closely conforms to the principle above? ['Motor vehicles are allowed to emit certain low levels of pollution; the resulting health risks are acceptable because all users of motor vehicles share the resulting benefit of inexpensive, convenient travel.', 'A motorcyclist rides without a helmet; the risk of fatal injury to the motorcyclist thus incurred is acceptable because the motorcyclist incurs this risk willingly.', 'A smoker subjects people to secondhand smoke at an outdoor public meeting; the resulting risks are acceptable because the danger from secondhand smoke is minimal outdoors, where smoke dissipates quickly.', 'A nation requires all citizens to spend two years in national service; since such service involves no risk to life, the policy is acceptable.']