What is the burden of proving beyond a reasonable doubt on causation in a murder case?

[2 Cal.App.4th 671] The state bears the burden of proving beyond a reasonable doubt every element of the offense charged. (In re Winship (1970) 397 U.S. 358, 364, 90 S.Ct. 1068, 1072, 25 L.Ed.2d 368.) Here, the jury was correctly instructed, "To constitute murder there must be, in addition to the death of a human being, an unlawful act which was the proximate cause of that death." Thus, although the jury was instructed it must find defendant's act was the proximate cause of the officers' death, the only evidence on causation it was allowed to consider was the prosecution's. A defendant cannot, constitutionally, be denied the right to present probative evidence rebutting an element of the crime. (People v. Wetmore (1978) 22 Cal.3d 318, 321, 149 Cal.Rptr. 265, 583 P.2d 1308.) By ruling the defendant's evidence as to causation inadmissible, the trial court effectively took the determination of proximate cause away from the jury. (Cf. People v. Marsh (1962) 58 Cal.2d 732, 741, 26 Cal.Rptr. 300, 376 P.2d 300.) It is no answer to say the state's evidence, standing alone, was sufficient to establish the element of causation beyond a reasonable doubt where the evidence which might have created a reasonable doubt was excluded.