因为that作代词用一般代替前面的不可数名词或可数名词单数,而those代替的是复数,这里代替cars是复数啊,
The weather in Zhejiang is hotter than that in Beijing （这时用that,代替不可数名词weather）如果是population的话就用that 应为population是不可数名词谢谢是的