According to SCOTUS in North Carolina V

According to SCOTUS in North Carolina v. Alford , which is true regarding a plea of guilty?

A) A defendant's plea of guilty, when the defendant believes he or she is innocent, can never constitute a voluntary guilty plea.
B) The only constitutional requirement is that the guilty plea be voluntary.
C) A guilty plea must consist of both a waiver of trial and an express admission of guilt.
D) To pass constitutional muster, a guilty plea must be voluntary and intelligent.