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
It is hard to tell whether we are going to have a boom in the economy or a （）

A. concession
B. recession
C. submission
D. transmission
单项选择题
The English vocabulary is known for a(an) ______ of words which are comparatively seldom used in ordinary conversations.
A. multitude
B. altitude
C. latitude
D. platitude
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