单项选择题

The word "it" (line 5, para 1) refers to______.

A．light bulb
B．energy
C．molecule
D．atom
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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
单项选择题
Townes's early work with stimulate emission was done with microwaves because______.
A．he was not concerned with amplification
B．his partner Schawlow had already begun work on the laser
C．it was easier to work with longer wavelengths
D．the laser had already been developed
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