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
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
单项选择题
According to the passage, the first maser was built probably in______.
A．1917
B．1951
C．1957
D．1953
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