The Socially Optimal Level of Provision of a Public Good

The socially optimal level of provision of a public good is where the

A) marginal cost of the output is equal to the vertical sum of the individual demand curves.
B) marginal cost of the output is equal to the horizontal sum of the individual demand curves.
C) marginal cost of the output is equal to the sum of the market's marginal benefit.
D) market price of the output is equal to the vertical sum of the individual demand curves.
E) market price of the output is equal to the vertical sum of all the market demand curves.