69. D 70. B 71. A 72. D
69.这是一道细节推断题。根据第一段“It has been pointed out that human beings， by agreement， can make anything stand for anything.”可推断出人们发出的各种噪音代表着任何一种活动。
70.这是一道主旨题。第二段“so the fact of being hungry can be symbolized by a thousand different noises according to the culture we live in”可知，我们的文化决定了某一种系统代表着什么。
71.这是一道猜测词义题。根据上文中“they are actually not so obvious as they seem except”可猜测出take special pains的意思是“努力尝试”。
72.这是一道主旨题。根据最后一段的内容可知，作者举小孩的例子说明人们有时对语言的作用产生误解。