According to The College Board website, 25%–35% of the multiple-choice questions are related to the concept of pollution (pollution types, impacts on environment and human health, and economic impacts such as cost-benefit analysis, externalities, marginal costs, and sustainability). If the test is coming up soon, focus there. The other core concepts – Earth Systems and Resources, the Living World, Population, Land and Water, Energy Resources and Consumption, and Global Change – all have approximately equal coverage percentiles, somewhere between 10 -15%. It is definitely worth it to spend some time focusing on pollution (pull up some videos, listen to the news, etc.).