do all/every thing (that) sd can(do)与do what sb can(do)的疑问

do all/every thing (that) sd can(do)与do what sb can(do)的疑问
这两句是省略句，后面跟不定式表目的。
如We must do everthing we can_pollution_happy.A.to prevent ; to live B,prevent；to live .....应选A。
而：I spend as much time as i can_both in and out of class.的缺空中为什么填的是practising,而不是to practise?
这两句都是省略句么？不是都是不定式做目的状语么？怎么会有后面的这种答案？还有：后面句子中的as much as i can 是宾语从句么？请细讲后句的各种结构。