白勇语法第5章语法第34练习题，高手帮助

34.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

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

因为all one has to do 不是正确的搭配,所以我认为b,c两项不正确,并且be动词后不能紧跟动词原型leave,所以我选了e

但答案选c,解释是all后的定语从句省略关系代词that,be动词后的不定式省略to.

知其然,不知其所以然,尤其是为什么be动词后有不定式的省略啊?

请NN解释一下吧?