Which Statement Describes a Positive Externality

Multiple Choice

Which statement describes a positive externality?

A) Sam dug a pond,so he could go fishing,but the pond has contributed to an explosion of mosquitoes in your neighborhood.
B) Sam has dozens of cats,and they come into your yard to hunt the birds that come to your birdbath.
C) Sam buys a dilapidated house,renovates it,and increases the property values of all the houses in the neighborhood.
D) Liquid waste from Sam's chicken farm flows into a neighbor's well water.