. It is difficult for us to learn a lesson in life we’ve actually had that lesson.
A. until
B. after
C. since
D. when
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(答案→)A. until
B. after
C. since
D. when
解析：until的意思是“直到……时”“在……之前”，如：We always have to wait until the last customer has left. 我们总得等着最后一位顾客离去。