Adam and Sam were engaged in fraud. The prosecution has reached a plea deal with Sam in exchange of his testimony against Adam. At Adam's trial, after Sam's testimony, the defense wants to impeach him by introducing the plea deal. The prosecution objects. If you were the judge, you should rule:

- The defense cannot introduce the plea deal for it has nothing to do with Sam's credibility.

- The defense may introduce the deal under impeachment by showing untruthful character.

- The defense may introduce the deal under impeachment for bias.

- The defense may introduce the deal under impeachment by contradiction.