单项选择题

This is an illness that can result in total blindness ______ left untreated.

A．if
B．after
C．since
D．because
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单项选择题
We have got only five textbooks for the children ______ we need ten. So I'm afraid they will have to share.
A．because
B．since
C．when
D．once
单项选择题
Mr. Hal! understands that ______ maths has always been easy for him, it is not easy for the students.
A．unless
B．since
C．although
D．when
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