单项选择题

Unless astronomers ______ their search for extraterrestrial life to stars as exceptional as the Sun, they are wasting much of their time.

A．narrow down
B．boil down
C．tone down
D．let down
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单项选择题
The assessment center gives each applicant the opportunity to ______ whether they are suited to the work.
A．exemplify
B．demonstrate
C．expose
D．exhibit
单项选择题
Next time you have a problem, think about how you can improve the situation instead of ______ all the negative aspects.
A．dwelling on
B．counting on
C．barking on
D．touching on
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