单项选择题

Some people viewed the findings with caution, noting that a cause-and-effect relationship between passive smoking and cancer remained _________.

A．to be shown
B．to have been shown
C．to have shown
D．being shown
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单项选择题
Over the years, Jimmy Connors __________ phenomenal displays of tennis and temper—and at the U.S. Open last week, he exhibited both again.
A．has treated spectators with
B．has treated spectator for
C．has treated spectators
D．has treated spectators to
单项选择题
_________ last year and is now earning his living as an advertising agent.
A．He would leave school
B．He left school
C．He had left school
D．He has left school
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