The Concept of a "Sensitive Period" for Language Learning in Humans

Multiple Choice

The concept of a "sensitive period" for language learning in humans means that:

A) if language is not learned by a certain age, it will never be learned
B) first languages can be learned at any age, but second languages must be acquired by middle adulthood
C) if a first language is not learned by a certain age, language acquisition will become more difficult
D) vocabulary can be learned at any age, but there is a small window during development when grammar can be acquired