单项选择题

I slipped and fell while I______.

A．am cleaning windows
B．clean windows
C．was cleaning windows
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单项选择题
The lady has been asking her boss ______two days off since last week.
A．with
B．/
C．for
单项选择题
Great changes______ place since he left here.
A．have been taken
B．have taken
C．were taken
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