How Are the Speech Adjustments Characteristic of Infant-Directed Speech Related

How are the speech adjustments characteristic of infant-directed speech related to children's grammatical development?

A) The more skilled a mother is at using infant-directed speech, the more advanced are her infant's grammatical skills.
B) The more infant-directed speech a mother addresses to her child, the less advanced her child's grammatical skills are likely to be.
C) Infant-directed speech is likely to harm a child's grammatical development unless the child receives other kinds of language input to compensate.
D) Researchers have yet to establish a strong link between infant-directed speech and children's grammatical development.