What constitutes a "greenhouse gas"?

A greenhouse gas refers to gases that trap heat in the atmosphere, contributing to the greenhouse effect. This effect is a natural process wherein certain gases, such as carbon dioxide, methane, and nitrous oxide, absorb and emit infrared radiation. By doing so, these gases help keep the Earth warm enough to sustain life, but excessive concentrations can lead to global warming and climate change.

In contrast, gases responsible for air pollution may include a variety of chemicals that do not necessarily contribute to heat retention, thus distinguishing them from greenhouse gases. While some gases emitted during natural disasters may include greenhouse gases, they do not define what a greenhouse gas is. Additionally, gases that contribute to acid rain are typically different from greenhouse gases and are related to the formation of sulfuric and nitric acids rather than heat retention. Thus, the primary characteristic of greenhouse gases is their ability to trap heat, which is accurately captured in the correct choice.