Who is the presiding officer of the senate? Who is the official chair, and how is that person traditionally chosen?

Karin bought illegal firearms at a gun show. at her trial, she alleged that she had committed this crime because her boyfriend had threatened to harm her and her two daughters if she did not. her lawyer asked the judge to instruct the jury that the prosecution had an obligation to prove beyond a reasonable doubt that karin had acted freely. instead, the judge told the jury that karin had the burden of proving duress by a preponderance of the evidence. who is correct?