单项选择题

In this research at Columbia University, Charles Townes worked with ______.

A．stimulated emission
B．microwaves
C．maser
D．stimulated emission and microwaves
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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 research work is still incomplete.
单项选择题
The word intervention (Line 3, Para. 1) can best be replaced by ______.
A．need
B．device
C．influence
D．reproduction
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