Given the positive externalities of more people having flu shots, if the government wants to push output closer to the socially desirable level, then one efficient policy would be to provide a subsidy to any citizen who wishes to get vaccinated. To be efficient, this subsidy should be:

A. more than the production cost of each flu shot.
B. equal to the vertical distance between the private demand curve and social demand curve.
C. less than the production cost of each flu shot.
D. equal to the horizontal distance between the private demand curve and social demand curve.