__________ are intestinal parasites that can live up to 20 years and grow up to 50 feet in length.

Tapeworms are intestinal parasites that can grow to about 50 feet long and may live for many years, even up to 20 years. Toxoplasmosis and malaria are diseases caused by parasites, while amoebae are single-celled organisms. Therefore, the correct choice is D, tapeworms.

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