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

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

A．stimulated emission
B．microwaves
C．maser
D．stimulated emission and microwaves

单项选择题

According to the passage, which of the following is TRUE of male dik-dik

A．He can in principle defend more than one female.
B．He practices monogamy.
C．He invests heavily in his offspring.
D．He is partly monogamous and partly polygamous.