Why Is the Social Marginal Benefit of a Common Property

Multiple Choice

Why is the social marginal benefit of a common property smaller than the value that people,on average,receive from it?

A) Because use of a common property is nonrivalrous.
B) Because when one additional person uses the common property,it lowers the value that others receive from it.
C) Because entrance fees must be taken into account when determining the social benefit derived from a common property.
D) Because "free riders" will use the common property without contributing to its costs.