单项选择题

A．Scientists thought there wasn’t.
B．Scientists hadn’t got an answer.
C．Scientists were uncertain.
D．Scientists were still studying.
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单项选择题
A．The gravitational force.B．The electromagnetic force.C．The weak force.D．The strong force.
A．The gravitational force.
B．The electromagnetic force.
C．The weak force.
D．The strong force.
单项选择题
A．David had difficulty in getting tickets.B．David has gone to the exhibit.C．David tried to get tickets after work.D．David will go to work the day after tomorrow.
A．David had difficulty in getting tickets.
B．David has gone to the exhibit.
C．David tried to get tickets after work.
D．David will go to work the day after tomorrow.
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