9. What conclusion can be made based on the results of the studies in the passage?
(A) Auditory input may not be a prerequisite for learning another language.
(B) Visual information about speech may play a critical role in language learning.
(C) Language is one trait that separates adults from babies who have not yet learnedto talk.
(D) Talking face-to-face to a baby triggers the development of language preferences.