单项选择题

Which of the following statements best describes a laser

A．A device for stimulating atoms and molecules to emit light.
B．An atom in a high-energy state.
C．A technique for destroying atoms or molecules.
D．An instrument for measuring light waves.
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单项选择题
Why was Towne's early work with stimulated emission done with microwaves
A．He was not concerned with light amplification.
B．It was easier to work with longer wavelengths.
C．His partner Schawlow had already begun work on the laser.
D．The laser had already been developed.
单项选择题
The writer considers that the Bronze Age succeeded the Stone Age because
A．there was a scarcity of wood and other building materials
B．bronze was less expensive than stone
C．stone was no longer available
D．more bronze than stone came into use
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