单项选择题

In approximately what year was the first maser built

A．1917.
B．1951.
C．1953.
D．1957.
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单项选择题
Why do people still argue about who deserves the credit for the concept of the laser
A．The researchers' notebooks were lost.
B．Several people were developing the idea at the same time.
C．No one claimed credit for the development until recently.
D．The work is still incomplete.
单项选择题
In his research at Columbia University, Charles Townes worked with all of the following EXCEPT _________.
A．stimulated emission
B．microwaves
C．light amplification
D．a maser
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