let me give it a try...
although I cannot proove why A has to be the right ans may be I can eliminate the rest and even B...
''singing'' is an action ... now when it comes to an action and comparision of actions of two different persons as is the case above... the use of ''do'' in neccessary...
like'' she can sing better than I do'' or ''she sings better than I do'' the use of ''can'' here creates the confusion... and when B says ''she can sing better than I '' I guess we cannot assume ''can'' here...
as for the second part of the B choice it is right...
''pretty'' is a fact or a feature and not an action... and so does not require the use of ''DO''
ex: she is taller than I ... now being tall is more of a fact than of an action.. and hence just simply putting or having ''I '' implies ''I am'' thr
whereas an action ceratinly needs a referent which is either ''does'' or do'' and B does not have that nor ''do'' any of the other choices above have.... U see I have used the ''do'' even here to compare ..cos I have used ''other choices HAVE ''and ''have'' indiactes ''possesion'' which is an action...
hope I was clear... don know If I was... but it was based purely on a little of basics and a lot on instinct so bare with me ...better info' welcome
Have fun