choice d:
'allows that it can...' is an incorrect idiom.
'...and so', while not ungrammatical, departs somewhat from the intended meaning of the original: so that (the original construction) implies a sense of purpose, intent, or design, while and so merely implies consequence, whether designed or arbitrary. for instance:
jim left his parents' back door open so that his friends can come take the liquor --> jim left the door open with this particular nefarious purpose in mind
jim left his parents' back door open, and so his friends can come take the liquor --> jim happened to leave the back door open, an oversight allowing his less straitlaced friends access to the alcohol
-- by 会员 19900620 (2012/4/26 14:25:39)