In many families adults speak to babies in simplified language . Yet constantly repeating simple phrases like "Nice kitty. See the kitty? " does not provide extra help to children in learning a language. We know this because there are families in which no one speaks to babies this way, yet the children in these families master the grammatical structure of their language just as well and as quickly as other children do. Which one of the following is an assumption on which the argument depends? ['Any child who has mastered the grammatical structure of a language has learned the language.', 'Many linguists believe that speaking to babies in simplified language helps the babies learn language.', 'Speaking to babies in simplified language could impair their language learning.', 'To learn a language one must acquire its vocabulary as well as its grammatical structure.']