What Can We Conclude About Age of Acquisition and Mastery

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

A) We cannot draw any conclusions until we have researched 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.