单项选择题

The word "intervention" (Line 3, Para. 1) can best be replaced by ______.

A．need
B．device
C．influence
D．reproduction
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单项选择题
Which of the following statements best describes a laser
A．A device for stimulating atoms and molecules to emit light.
B．An atom in a high-energy state.
C．A technique for destroying atoms or molecules.
D．An instrument for measuring light waves.
单项选择题
In this research at Columbia University, Charles Townes worked with ______.
A．stimulated emission
B．microwaves
C．maser
D．stimulated emission and microwaves
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