一道两个选项貌似都对的gmat语法题

一道两个选项貌似都对的gmat语法题
Researchers hypothesize that granitic soil is the ideal construction material for the desert tortoise because it is not so hard that it makes burrowing difficult or so soft that it could cause tunnels to collapse
A. so hard that it makes burrowing difficult or so soft that it could cause
B. hard enough to make burrowing difficult or soft enough as to cause
C.So hard as to make burrowing difficult or soft enough so it causes
D. as hard as to make burrowing difficult or as soft as to cause
E. too hard, making burrowing difficult, nor too soft, so as to cause
哪位高手可以讲解一下A和D吗?谢谢了!(答案为a,但我不知道d为什么不对）