5. The brain finds it best to keep smell receptors available for unfamiliar and emergency signals such as the smell of smoke, which might indicate the danger of fire.

【分析】复合句。句子主干为 The brain finds it best to keep smell receptors available。其中 it 为形式宾语，不定式结构 to keep smell receptors available 才是真正的宾语，best 为宾语补足语；such as... 用于举例，补充说明 unfamiliar and emergency signals；which 引导的定语从句修饰 the smell of smoke。

【译文】 大脑发现最好让气味感受器随时用于接收陌生的和危急的信号，比如可能暗示火灾危险的烟味。

2004年

1. Many theories concerning the causes of juvenile delinquency (crimes commited young people) focus either on the individual or on society as the major contributing influence.

【分析】简单句。句子主干为 Many theories...focus either on the individual or on society。concerning the causes of juvenile delinquency 是分词短语作后置定语，修饰 theories；括号内的成分是 juvenile delinquency 的同位语，对其进行解释； as the major contributing influence为宾语补足语。

【译文】 很多关于青少年犯罪（低龄人群犯罪）原因的理论要么强调个人要么的强调社会是导致犯罪的主要原因。

【点拨】1) juvenile 既是名词“青少年”，也是形容词“青少年的；年幼无知的”。2) commit“委托，托付；犯罪，承诺，做...事”。3) influence 原意为“影响；作用”，本句中意为“影响因素”。