Because Ryan got a flu vaccine, his roommate Mitch who had a low immune system was spared from getting the flu. This is even without Mitch paying for and getting a vaccine. This implication in this specific story is that because of

a. the negative externality from a vaccine, the vaccine service seller should have been compensated 2x what it received in payment

b. the negative externality from a vaccine, the vaccine service seller should have been compensated 4x what it received in payment

c. the positive externality from a vaccine, the vaccine service seller should have been compensated 3x what it received in payment

d. the positive externality from a vaccine, the vaccine service seller should have been compensated 2X what it received in payment

e. the positive externality from a vaccine, the vaccine service seller should have been compensated 20X what it received in payment