Research on Learning a Second Language

Research on learning a second language:

A) has proven that the critical period for learning a second language ends around age 8.
B) has proven that the critical period for learning a second language ends at the onset of puberty.
C) suggests there is at least a sensitive period for learning a second language,but when the sensitive period ends is unclear.
D) suggests that neither sensitive nor critical period exists for learning a second language.