因为one是同类事物不同地点不同所有权时候的指代.代词that指代的是同一个物品,或者是独一无二的无法区分相同或者不同名词.比如the weather here is hotter than that of Beijing.可用来代替前面提到的事物（单数名词或者不可数名词）,以避免重复这个名词,但必须是这一名词在第二次出现时有后置定语.如：
Their cotton output (产量) of 1999 was double that of 1998 .
The sweater he wears today is different from that he wore yesterday .
The study of idiom is as important as that of grammar .