_____pollution is the key to ___the country a comfortable and clean place to live in.
问题描述：
_____pollution is the key to ___the country a comfortable and clean place to live in.
为什么两个空分别是To control;keeping而不是Controlling;keep?
答
1)动词不定时表目的
2)“the key to ”中的“ to ”是介词,即“the key to sth/doing”