The Reason Why We Sum the Demand Curves of Individual

The reason why we sum the demand curves of individual consumers vertically rather than horizontally in a public goods market is that

A) the willingness to pay in a public good market is smaller than that in a standard market.
B) individual consumers are unlikely to be willing to pay enough to cover the marginal cost of production of a public good.
C) the marginal social benefit of the X^th unit includes all consumers' marginal social benefits because the public good is non-exclusive and non-rival.
D) the concept of quantities is meaningless in the case of public goods.