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
In conclusion, walking is a cheap, safe, enjoyable and ______ available form of exercise.
A．recklessly
B．readily
C．potentially
D．amenably
单项选择题
The ______ amongst the world's scientists is that the world is likely to warm up over the next few decades.
A．censure
B．consensus
C．census
D．censor
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