问题

I didn’t manage to do it _____ you had explained how.
A:until
B:unless
C:when
D:before
B:unless
C:when
D:before
正确答案是:
同类题型
Jim couldn’t understand how the machine worked until his friends____ how to use it.
Is the Great Wall a wonderful place?—Yes, tourists come here for a visit every year.
Have you seen Mr. Black these days, Jack? —-Yes, I met him when I ________ in the street yesterday.