so..as to VS SO ..THAT

The Emperor Auguest, it appears ,commisioned an idealized sculptured portrait ,the features of which are so unrealistic as to constitute what one scholar calls an "artiticial face".

a) so unrealistic as to constitute

b) so unrealistic they constitute

c) so ounrealistic that they have constituted

d) unrealistic enough so that they constitute

e) unrealistic enough so as to constitute

the correct answer is a) why not c).

因为so..that前后两部分时态不同，故不能用SO。。AS TO ，因选C