单项选择题

Scientists say it may be five or ten years ______ it is possible to test this medicine on human patients.

A．since
B．when
C．after
D．before
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单项选择题
Everyone who has visited the city agrees that it is ______ with life.
A．vibrant
B．violent
C．energetic
D．full
单项选择题
No matter how frequently ______, the works of Beethoven always attract large audiences.
A．performed
B．performing
C．to be performed
D．being performed
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