Johnson and Newport's Study of the English Proficiency of Korean

Johnson and Newport's study of the English proficiency of Korean and Chinese immigrants in the United States demonstrated that knowledge of English grammar was associated with the

A) length of time individuals had lived in the United States.
B) age at which individuals began learning English.
C) individuals' motivation to learn English.
D) length of time individuals had lived in the United States and the age at which individuals began learning English.