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

As per the question, the correct revision of the given phrase is displayed through 'option A' as it correctly employs the idiomatic phrase and uses 'that' correctly to indicate the results or outcomes('So hard that it makes burrowing difficult' or 'so soft that it could cause') of an action. The other options either carries parallelism error in presenting the degrees equally('hard enough' and 'soft enough as' in option B', 'so hard as' and 'so enough so' in option C) or fails to portray the comparison inappropriately(options D(no use of noun after 'as hard as' to which it compares and in option E('too hard' to what is not elaborated). Thus, option A is the correct answer.