The answer is C, Balantidium coli.  Although it has been reported in many different simian hosts, domestic hogs are probably the most important reservoir host for human infections.  The incidence of human balantidiasis tends to be highest in areas where pigs are common domestic animals.  Balantidium coli is also the largest intestinal protozoa that infects people, and can measure up to 100 µm in diameter.

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