What can we conclude about the relationship between a person's age of acquisition of a second language and his or her mastery of grammar?

a. So far, no research has demonstrated a relationship between these two variables.
b. Linguistically skilled individuals tend to master grammar better if they learn a second language during childhood; individuals with below-average skills tend to master grammar better if they learn a second language during adulthood.
c. The results clearly support the critical period hypothesis, because people have greater mastery of grammar if they learn a second language during childhood.
d. The results are complicated, because the answer depends on the match between the two languages, as well as the amount of education in the second language.

Ans: d