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 hope to go to the beach tomorrow, but we won't go ______ it's raining.
A．if
B．when
C．though
D．because
单项选择题
The main reason for the population growth isn't so much a rise in birth rates ______ a fall in death rates as a result of improvements in medical care.
A．and
B．but
C．or
D．as
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