GWD语法一题， 答案不确定 请教中

xuromeo

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
为什么答案A当中前面是MAKES 后面确实COULD 好像不平行啊？
求解

yeah529

B hard enough to...soft enough as to不平行。。
C so hard as to....soft enough so..不平行
D 选项改变了句意。。而且it is as hard as to make...or as soft as to cause不对。要平行的话前面也应该是不定式的结构，比如to do sth is as hard as to make这样。
E 结构凌乱- -too hard, 后面加现在分词 然后too soft后面又加so as to。