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
People suffering from ______ prefer to stay shut in their homes and become panic-stricken in large public buildings and open fields.
A．acrophobia
B．agoraphobia
C．claustrophobia
D．xenophobia
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单项选择题
Too much ______ can possibly lead to unhappiness, even to thoughts of suicide as few people have the courage to analyze themselves objectively and minutely.
A．retrospect
B．introspection
C．perspicacity
D．perspicuity
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