What can we conclude about age of acquisition and mastery of a second language?

a. We cannot draw any conclusions until we have research using the experimental method.
b. For speakers of first languages similar to English, it's better to learn English early; for speakers of first languages that are different from English, it's better to learn English late.
c. The research shows quite clearly that children acquire a second language more rapidly than adults do.
d. The research is not clear-cut; instead, the findings seem to depend upon factors such as the similarity between the first language and the second language.

Ans: d