Which of the following is the best example of a positive externality?
a. A car company lays off 10,000 employees in order to cut costs.
b. An individual drives a large SUV, which increases the amount of greenhouse gases in the atmosphere.
c. An individual smokes in a restaurant, which bothers other patrons.
d. A child is vaccinated against a disease, which reduces the probability that other children will catch the disease.
