单项选择题

Leaders from all over the world ______ together at the United Nations since 1952.

A．work
B．are working
C．have worked
D．have been working
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单项选择题
Carter agreed to try to reduce American oil imports and control ______ in the United States.
A．infection
B．inflation
C．inspection
D．inspiration
单项选择题
There were a number of people already ______ on the platform.
A．sit
B．to sit
C．sitting
D．having sat
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