单项选择题

You ______ see the doctor if that back ache persists.

A．better
B．better have
C．have better
D．had better
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单项选择题
I've worked here since I ______ Harvard Business School.
A．leave
B．left
C．have left
D．had left
单项选择题
Do earthquakes ______ often in North America
A．break
B．break out
C．happen to
D．occur
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