Suppose you have a 22-year-old friend who came to North America from Korea when he was 15 and then began to learn English. If he is like the participants in Flege's study on bilingualism, he will find that

a. he will develop trouble in Korean with verb tenses and the gender of nouns, but will maintain his Korean vocabulary.
b. he will lose a substantial percentage of his Korean vocabulary.
c. he will have difficulty mastering English vocabulary.
d. he will master English grammar as well as a Korean speaker who learned English as a 10-year-old child, as long as they both have the same amount of U.S. education.

Ans: d