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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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
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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