What is the state prosecutor's argument that only the testimony of a single witness is sufficient for the proof of any fact?

The prosecutor's explanation also had a basis in trial strategy. Because the case turned, in large part, upon the testimony of a single witness, the prosecutor's trial strategy would naturally be to find jurors who could base their decision on the testimony of a single witness. (See People v. Avila (2009) 46 Cal.4th 680, 703 ["'the testimony of a single witness is sufficient for the proof of any fact'"]; see also CALCRIM No. 301.) Jurors that would have trouble doing so would be the focus of peremptory challenges.