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？）
选项：

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）