求教CR一个题干里面的语法知识

maximilien

cr1 17   Urban air contains more sulfur dioxide than does rural air, and plants in cities typically grow more slowly than do plants in rural areas. In an experiment to see how much of the difference in growth is due to sulfur dioxide, classes in an urban and a rural school grew plants in greenhouses at their schools and filtered the greenhouse air to eliminate sulfur dioxide.  lants in the urban greenhouse grew more slowly than those in the rural greenhouse.这道题没什么逻辑上的问题，其实是想问问语法，划线的两个部分，does和do为什么放在前面？一般会写成urban air contains more sulfur dioxide than rural air does....吧，求教这个语法点。谢谢。

TerraceHo

補充helping verbs的目的，在於突出than後面的noun是subject而不是object，不要引起歧義。順序應該是沒關繫的

kiwifoodtown

比较结构中，常用倒装形式。

maximilien

谢谢楼上两位