TOEFL Listening: ETS-TOEFL听力机经 - F94S0930U30T3I8ZP$

What can be inferred about auto-grooming? A. It does not really contribute to the animals' hygiene. B. It's purpose is more limited than the purpose of allo-grooming. C. It has been researched more thoroughly than allo-grooming. D. It leads to the release of endorphin into the animals' bloodstream.