Choose one answer. All questions are included, with explanations after checking. No time limit. Training target: 20/24. XSD 1.0 identity constraints: missing and composite fields, typed equality, same-owner key references, repeated-container scope and namespace-aware selectors. Assumes basic XSD knowledge. Excludes XSD 1.1 assertions and cross-level keyref visibility. Sources: https://www.w3.org/TR/2004/REC-xmlschema-1-20041028/#Identity-constraint_Definitions https://www.w3.org/TR/2004/REC-xmlschema-0-20041028/#specifyingUniqueness
