Question

moreover, animals eating mostly snails in the more developed monterey bay were more likely than others to contract the parasite toxoplasma gondii. researchers dont know how otters encounter the parasite, but its found in cat feces, and they speculate it could wind up in the ocean when cat litter is flushed down the toilet or when rain washes feral or domestic cat waste into storm drains and the bay. in contrast, otters that consumed mostly clams and worms were more likely to be infected with sarcocystis neurona; the source of this parasite in the otters environment is also not known, but its found in opossum feces and could possibly be washed into the bay by rainwater. otters may have little natural immunity to these terrestrial pathogens, which can kill them outright or impair their ability to swim, possibly leaving them more vulnerable to sharks.
8 “we absolutely think those parasites are getting into the ocean in much greater numbers due to the way we modify the landscape,” says tinker. “all the bacteria and viruses that were producing with a couple miles of the oceans are being washed directly into that near - shore area, and we think sea otters are bearing the brunt of that.”
9 in that sense, the california sea otter is not just a keystone of the marine environment but also an indicator. if otters are healthy, the ocean probably is, too. when the otter population is struggling, the near - shore waters they inhabit may be in trouble. “in terms of sea otters, you are what you eat,” says david jessup, a veterinarian at the marine wildlife veterinary care and research center in santa cruz. “at least here in california.”
“picky eaters,” by jess righthand © 2011. originally published in smithsonian magazine. used by permission of the author.
why does the author use the metaphor of the keystone in describing the sea otter?
a to spotlight the unusual diet of the sea otter
b to illustrate the sea otters distinctive abilities
c to suggest that the sea otter has many predators
d to stress the sea otters significance to the environment

Explanation:

The text mentions that the California sea - otter is a keystone of the marine environment and an indicator of the health of the near - shore waters it inhabits. The metaphor of the keystone is used to emphasize its significance to the environment.

Answer:

D. to stress the sea otter's significance to the environment