TOEFL Listening: ETS-TOEFL听力机经 - 5JO787JS75MXU9Z7P$

What type of genetic mutations occurred in the evolution of certain animals that enabled them to become immune to tetrodotoxin? A. Mutations that prevent the toxin from blocking sodium ion transmissions B. Mutations that help the respiratory system fight off invasion by bacteria C. Mutations that prevent the animals from producing excessive amounts of the toxin D. Mutations that allow the animals to exist in bacteria-free environments