Mark Is on Trial for Murder of Victor and Has

Mark is on trial for murder of Victor and has hired Anthony, an attorney, to represent him. He claims he acted in self-defense. Mark's sister Sandy was present when the crime occurred. Anthony told Mark that Sandy would be impeached if she took the witness stand. Why would Anthony say this?

A) Sandy may be biased because she is the defendant's sister
B) Sandy is a federal judge
C) Sandy is a minor
D) Sandy has never testified at a trial before