Economists Tend to Distrust Voluntary Approaches as a Way to Deal

Economists tend to distrust voluntary approaches as a way to deal with externalities. What is their most common concern?

A) Voluntary approaches do not make people develop an awareness of the problem that would lead them to make good lifestyle changes.
B) Voluntary approaches often are perceived as unfair, imposing a heavy burden on the poor.
C) Voluntary approaches usually require people to ignore their self-interest, and economists do not think people do that well.
D) Voluntary approaches are often too effective and lead to excessive reduction in the externality.