单项选择题

Experts say children can begin learning a second language in preschool, and should begin be- fore the age of ten to take advantage of young children’s natural ability to acquire language. This advantage is lost by the time most children encounter a foreign language in the classroom, in middle school or high school.
Dr, Noble Goss, an assistant professor of Spanish’ and German at Harding University in Arkansas, says, as a father of bilingual (双语的) children (My wife is Mexican), I know that not only it is easy for children m learn one or more languages, but they have an amazing way of separating them out more successfully than you would think.
If a child hears two languages early, he accepts both equally. But if given a chance to speak both, can they be fluent in both
Not everyone agrees that early childhood is the only time to start a language, however. Reeves, the chairman and founder of the Center for Performance Assessment in Denver, Colorado, points to the Defense Language lnstitute’s success at training adult students to speak other language fluently.
The frequently quoted argument that post-adolescence (青春期后) is too late to acquire foreign language skills is still a myth, says Reeves.
Still, if children are going to devote the six years or so of study it takes to master a second language, starting early just makes sense.

Experts advise starting a foreign language early mainly because（）

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