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