Ocean Acidification Practice Test

Two coastal regions experience changes in water chemistry. Region 1 shows a long-term decline in average seawater pH that tracks rising atmospheric $\mathrm{CO_2}$. Region 2 shows short-term pH drops immediately after heavy storms that wash pollutants from land into the ocean. Which statement best distinguishes ocean acidification from acid rain/runoff effects?

A coastal monitoring station reports that average surface-ocean pH near a reef declined from 8.12 in 1990 to 8.05 in 2020 while atmospheric $\mathrm{CO_2}$ increased over the same period. Which mechanism best explains this pH decline (ocean acidification)?