单项选择题

A new technique ______, the yields as a whole increased by 20 percent.

A．working out
B．having been worked out
C．having worked out
D．to have been worked out
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单项选择题
Even in ancient times, there were ______ of night-watchmen which went about the cities.
A．guards
B．teams
C．routes
D．patrols
单项选择题
Jack ______ from home for two days now, and I am beginning to worry about his safety.
A．has been missing
B．has been missed
C．had been missing
D．was missed
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