PREFIX : <http://example/>
PREFIX  n: <http://example/x#> 
PREFIX  rdf:    <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX  rdfs:   <http://www.w3.org/2000/01/rdf-schema#>
PREFIX  xsd:    <http://www.w3.org/2001/XMLSchema#>

SELECT *
{ 
    ?x rdf:type/rdfs:subClassOf* :B
}