单项选择题

(Those of us) who work in chemical (laboratories), should have (their) lungs (checked) quite regularly.

A. Those of us
B. laboratories
C. their
D. checked
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单项选择题
You should have been more patient ______ that customer; I'm sure that selling him the watch was a possibility.
A．of
B．with
C．for
D．at
单项选择题
The story was said to have been based on the information from a reliable ________.
A．source
B．foundation
C．origin
D．basis
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