Selection properties of the split interval and the Continuum Hypothesis

We prove that every usco multimap $\mathrm{\Phi }:X\to Y$ 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 $P:{\ddot{\mathbb{I}}}^2\to {\mathbb{I}}^2$ of its square onto the unit square ${\mathbb{I}}^2$, the usco multimap $P^{-1}:{\mathbb I}^2\multimap\ddot{\mathbb I}^2$hasaBorel($F_\sigma$-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.