When the Prosecuting Attorney Attempts to Introduce Evidence Obtained as a Result

When the prosecuting attorney attempts to introduce evidence obtained as a result of a consent to search into court evidence, which of the following does the court require?

A) the defense to prove beyond a reasonable doubt that the consent was involuntary.
B) the defense to prove that there is probable cause to doubt that the consent was involuntary.
C) the prosecutor to prove beyond a reasonable doubt that the consent was voluntary.
D) the prosecutor to prove by a preponderance of the evidence that the consent was voluntary.