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The presence of microorganisms that produce a toxin causes seawater to turn brownish red, a phenomenon known as a red tide. Sea otters do not feed in areas where clams, their main source of food, have become contaminated with this toxin. According to a proposed explanation of the otters' behavior, the otters sample the clams in a potential feeding area and can taste any toxin in them.

Which of the following , if true, would most strongly indicate that the hypothesis described in the last sentence of the passage is not correct?

A In some of the areas where red tides occur, neither clams nor sea otters are indigenous species

B The presence of sea otters in a given area has a significant effect on which other marine organisms are to be found in that area

C When seawater in an area unaffected by red tide is artificially dyed brownish red, sea otters do not feed on the clams in that area

D If the clams in a given area are contaminated with toxins, sea otters move to other areas in search of food.

E Although  very small amounts of the toxin produced during a red tide are not harmful, large doses can be fatal to animals the size of sea otters

I am confusing about choice C and D

Please help