What is the burden of proving beyond a reasonable doubt that a defendant's prior convictions were for either serious or violent felonies?

"The prosecution bears the burden of proving beyond a reasonable doubt that a defendant's prior convictions were for either serious or violent felonies. When a defendant challenges the sufficiency of the evidence to uphold a finding that his prior convictions qualified as strikes, the test on appeal is whether a reasonable trier of fact could have found that the prosecution sustained its burden. We review the record in the light most favorable to the trial court's findings. [Citation.]" (People v. Towers (2007) 150 Cal.App.4th 1273, 1277.)

Penal Code section 667, subdivision (d) states in part:

"(3) A prior juvenile adjudication shall constitute a prior felony conviction for purposes of sentence enhancement if: