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．When he was seventeen.B．Shortly before he died.C．When he was thirty.D．Two weeks before he died.
A．When he was seventeen.
B．Shortly before he died.
C．When he was thirty.
D．Two weeks before he died.
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.
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