单项选择题

Experiments in the photography of moving objects （） in both the united states and Europe before 1990.

A．have been conducting
B．were conducting
C．had been conducted
D．are conducted

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单项选择题
Agriculture was a step in human progress （） which subsepuently was not anything comparable until our own machine age.

A．in
B．for
C．to
D．from
单项选择题
（） have come up with the solutions to the problem.

A．The boy student
B．The boys student
C．The boy students
D．The boys sutdents
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