The HPV Vaccine Protects Against Human Papillomavirus (HPV) That Causes

The HPV vaccine protects against human papillomavirus (HPV) that causes cancer. It protects both the person getting the vaccine and the sexual partners of that person. To encourage the efficient number of people to be vaccinated the government could

A) give producers a subsidy equal to the private marginal benefit.
B) give consumers a voucher equal to the marginal external benefit.
C) impose a tax equal to the marginal social benefit.
D) impose a tax equal to the marginal external benefit.