The Marginal Social Benefit Curve for Water Quality Slopes Downward

The marginal social benefit curve for water quality slopes downward because _____

A) the total benefit from clean water decreases as water becomes cleaner.
B) the added benefit from cleaner water is greater when water quality is poor than when water quality is high.
C) the added benefit from cleaner water is smaller when water quality is poor than when water quality is high.
D) the total benefit from clean water increases as water becomes cleaner.
E) the average benefit of clean water is higher.