well IMO both B and C have a missing element :
B-->what is the selection policy of the party to which DA belonged when he joined ? will decide the new selection.
C--> what if the DA changed the affiliation during the chaired time, so the selection policy of the party to which he currently belongs will have no effect on the selection of the new DA.
but the construction of the argument towards the conclusion implicitly directs more towards C than B.
so C