To Find the Social Marginal Benefit of Public Goods, One

To find the social marginal benefit of public goods, one needs to:

A) sum the consumers' demand curves vertically.
B) sum the consumers' demand curves horizontally.
C) sum the marginal private benefit and the marginal external benefit for each unit.
D) sum the marginal private cost and the marginal external cost for each unit.
E) subtract the individual portion of the tax burden necessary for the government to provide the good from the demand curve of each consumer who desires the good.