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:

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