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、The relation between RDF Schema and RDF is similar as the relation between XML Schema and XML (RDF Schema 和RDF之间的关系类似于XML Schema与XML之间的关系);
B、RDF Schema is a semantic extension to RDF （RDF Schema是RDF的语义扩展）;
C、RDF Schema is also a RDF document （RDF Schema也是一个RDF文档）;
D、RDF Schema is the foundation of the Semantic Web （RDF Schema是语义网的基石）