单项选择题

Hydrocarbons, ______ by engine exhausts, react with nitrogen oxides in the presence of sunlight to form complex toxic gases.

A．are given off
B．give off
C．they are given off
D．given off
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单项选择题
It is ______ understood by all concerned that the word no one who visits him ever breathe a syllable of in his hearing will remain forever unspoken.
A．uncommunicatively
B．acceptably
C．tacitly
D．taciturnly
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