单项选择题

We shall probably never be able to ______ the exact nature of these sub-atomic particles.

A．assert
B．impart
C．ascertain
D．notify
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单项选择题
The magician made us think he cut the girl into pieces but it was merely an______
A．illusion
B．impression
C．image
D．illumination
单项选择题
So many directors ______, the board meeting had to be put off.
A．were absent
B．being absent
C．been absent
D．had been absent
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