___is done to clean the air.为什么填little不填a little

___is done to clean the air.为什么填little不填a little
___is done to clean the air.为什么填little不填a little

___is done to clean the air.为什么填little不填a little
little是相当于代词,a little是副词词组
much,enough,many,some都可以这样用
Enogh has been said on how to learn English well.
Much has been done to protect the wildlife.