单项选择题

The last half of the nineteenth century _________ the steady improvement in the means of travel.

A．has witnessed
B．witnessed
C．is witnessed
D．has been witnessed
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单项选择题
He wants to find a temporary job to fill the _________ between school and university.
A．gap
B．opening
C．interruptions
D．margin
单项选择题
It was not until midnight _________ the snowcapped peak.
A．that they sighted
B．that they did not sight
C．did they sight
D．had they sighted
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