单项选择题

In his research at Columbia University Charles Townes worked with all of the following EXCEPT______.

A．stimulated
B．amplification
C．microwaves
D．a maser
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单项选择题
The word it (line 5, para 1) refers to______.
A．light bulb
B．energy
C．molecule
D．atom
单项选择题
A laser can be best described by______.
A．a device for stimulating atoms and molecules to emit light
B．an atom in a high energy state
C．a technique for destroying atom or molecules
D．an instrument for measuring light waves
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