Which of the following statements about RDF Schema are correct? （下面关于RDF Schema的说法哪些是正确的？）

A、TherelationbetweenRDFSchemaandRDFissimilarastherelationbetweenXMLSchemaandXML(RDFSchema和RDF之间的关系类似于XMLSchema与XML之间的关系);
B、RDFSchemaisasemanticextensiontoRDF（RDFSchema是RDF的语义扩展）;
C、RDFSchemaisalsoaRDFdocument（RDFSchema也是一个RDF文档）;
D、RDFSchemaisthefoundationoftheSemanticWeb（RDFSchema是语义网的基石）