[求助]to到底省不省？

21. It is an oversimplified view of cattle raising to say that all one has to do with cattle is leave them alone while they feed themselves, corral them, and to drive them to market when the time is ripe.
(A) all one has to do with cattle is leave them alone while they feed themselves, corral them, and to
(B) all one has to do with cattle is to leave them alone to feed themselves, to corral them, and
(C) all one has to do with cattle is leave them alone while they feed themselves and then corral them and
(D) the only thing that has to be done with cattle is leave them alone while they feed themselves, corral them, and
(E) the only thing that has to be done with cattle is to leave them alone while they feed themselves, to corral them, and

这个题目中省略了to,

什么时候可以省略什么时候又不可以省略呢

请NN们解答下吧？