Why Do US Economists Commonly Refer to Externalities as an Example of Example

Multiple Choice

Why do U.S. economists commonly refer to externalities as an example of market failure?

A) firms that are required to pay social costs of externalities produce more
B) externalities present a case where markets consider all social costs
C) externalities present a case where markets only consider some social costs
D) firms avoid having to pay social costs of externalities by lowering prices