资料题：One of the great changes brought about by the knowledge society is that（　）。

The fridge is considered a necessity.It had been so since the 1960s when packaged goods first appeared with the label: "store in the refrigerator" .In my fridgeless childhood in the 1950s, I was fed well and healthily.The milkman came daily, the grocer, the butcher, and the ice - cream man delivered two or three times a week.The Sunday meat would last until Wednesday and surplus bread and milk became all kinds of cakes.Nothing was wasted, and we were never troubled by rotten food.Thirty years on, food deliveries have ceased, fresh vegetables are almost unobtainable in the country.The invention of the fridge contributed comparatively little to the art of food preservation.A vast way of well-tried techniques already existed-natural cooling, drying,smoking.salting,sugaring, bottling.Consequently, most of the word' s fridges are to be found, not in the tropics where they might prove useful, but in the wealthy countries with mild temperatures where they are climatically almost unnecessary.Every winter, millions of fridges hum away continuously, and at vast expense, busily maintaining an artificially - cooled space inside an artificially - heated house -while outside, nature provides the desired temperature free of charge.The fridge' s effect upon the environment has been evident, while its contribution to human happiness has been insignificant..If you don' l believe me, try it yourself, invest in a good cabinet and turn off your fridge next winter.You may miss the hamburgers, but at least you'll get rid of that terrible hum.