2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/139040We give, for each level of complexity L, a Hurewicz-like characterization of the Borel subsets with countable sections of a product of two Polish spaces that cannot become in L by changing the two Polish topologies.LogicGeneral Topology54H05, 03E15, 26A21, 28A05text