Pollution of the atmosphere as a result of vehicle emissions is an example of:

A) a commons dilemma.
B) the free-rider effect.
C) mass rapid transit systems.
D) insufficient research.

A commons dilemma occurs when individual users who have open access to a shared resource (such as air) act independently according to their own self-interest and contrary to the common good of all users, causing depletion or degradation of the resource. Vehicle emissions contribute to air pollution, exemplifying a situation where the atmosphere is treated as a common resource that is overused and degraded by individual actions.