单项选择题


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