Until about six months, infants can distinguish the:

1. Researchers found that from birth to six months infants can discriminate the phonetic contrasts of all languages. 2. After this age, the child perceives only the phonemes specific to the language being learned. 3. So early infancy is a sensitive period for sound discrimination. 4. Written symbols are irrelevant to babies, and the finding is not limited to the mother's voice. **Why not the others:** Home-language-only perception develops after six months, and written symbols and one voice are not what the finding is about. _Source: Wikipedia, "Language acquisition", section "Sensitive period" (revision 1375526787)_