Adefendant is on trial for manslaughter after he hit a victim in a bar, causing the victim to fall and hit his head on the marble bar top. the defendant claims that he hit the victim in self-defense after the victim lunged at him with a knife. during the prosecution's case, a witness testifies that she heard the victim's friend shout at the defendant, "you just killed a man! " a defense witness is called to testify that he was there and does not remember hearing the victim's friend say anything. should the defense witness's testimony be admitted? a no, it is irrelevant to any issue in the case. b yes, it is proper impeachment of the prosecution's witness. c no, it is improper impeachment of the prosecution's witness because it relates to a collateral matter. d no, it is improper impeachment because it does not positively controvert the prosecution witness's testimony, as the defense witness merely says he does not remember.