By the time ______, Jim _______ three hundred Chinese characters.
A. he was five; had learnt B. he was five; has learnt
C. at the age of five; learnt D. when he was five; has
知识点：过去完成时及其被动式
A. he was five; had learnt B. he was five; has learnt C. at the age of five; learnt D. when he was five; has