单项选择题

They repeated the experiment several times ______ they were sure that the result was reliable.

A．until
B．for
C．since
D．when
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单项选择题
Yesterday when I was walking on the street, I ______ to see an old friend of mine.
A．managed
B．happened
C．settled
D．occurred
单项选择题
I had hoped that I ______ the lecture on American Romanticism.
A．attend
B．attending
C．could attend
D．can attend
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