Selection properties of the split interval and the Continuum Hypothesis

We prove that every usco multimap $Phi:Xto Y$ from a metrizable separable space $X$ to a GO-space $Y$ has an $F_sigma$-measurable selection. On the other hand, for the split interval $ddot{mathbb I}$ and the projection $P:ddot{mathbb I}^2to{mathbb I}^2$ of its square onto the unit square ${mathbb I}^2$, the usco multimap $P^{-1}:{mathbb I}^2multimapddot{mathbb I}^2$ has a Borel ($F_sigma$-measurable) selection if and only if the Continuum Hypothesis holds. This CH-example shows that know results on Borel selections of usco maps into fragmentable compact spaces cannot be extended to a wider class of compact spaces.