Read the following passage:
California sea otters feed and live in the "near shore marine ecosystem"—the stretch of ocean that hugs a shoreline—making them a good indicator species for detecting pollutants and pathogens1 washed down from coastlands. And along many parts of Pacific North America, sea otters play critical roles in natural food webs, keeping important ecosystems like kelp forests and seagrass beds in balance. Studying sea otters, then, not only helps us understand their population recovery, but it also uncovers clues on the health and rhythm of near shore marine ecosystems—the same waters many people swim, fish, and make a living in every day.
According to the article "Slowly Swimming Toward Recovery, California's Sea Otter Populations Holding Steady," sea otters are important to protect because they
A . demonstrate how different pollutants affect populations
B . are loved by children all over the world
C . have experienced a variety of changes in population
D. play critical roles in natural food webs