system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - whatever happened to acid rain?
gpt
Acid rain is primarily caused by specific kinds of air pollutants. Legislation in North America and Europe successfully limited the emission of these pollutants--for example, through the establishment of a "cap and trade" system that limits the total pollution and requires companies to pay more to buy the a greater share of that cap. That's why it's not nearly as serious of a problem anymore in the West and you don't hear about it: the solutions are working.

In many industrializing countries, however, like China, acid rain is still a serious problem.