Certain species of aquatic organisms are particularly sensitive to environmental changes. Scientists often monitor the behavior, population size, and overall health of these so-called indicator species in order to evaluate the water quality in an ecosystem. Biological indicator species are important to water systems because
A.
they provide an early warning sign of degrading water quality in an ecosystem.

B.
they are used to control water quality by filtering out pollutants in an ecosystem.

C.
they can convert harmful pollutants into helpful nutrients in an ecosystem.

D.
they become healthier as water quality degrades and can replace weaker species.