Which of the Following Is NOT a Barrier to Reforming

Which of the following is NOT a barrier to reforming the current nomination sequence?

A) The public supports the present system and is opposed to change.
B) It is difficult for political parties to impose sanctions on state parties that disobey party primary rules.
C) If Congress imposed a national primary system, states would challenge its constitutionality.
D) States, not parties, control when primaries are held.