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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picnics[解析] 由卒格前的介词at，可以推断出此处需要一个名词作介词宾语。结合录音填入picnics。
单项选择题
It was not until bedtime ______ he hurried back to the dormitory.
A．when
B．unless
C．that
D．which
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