Get all root terms from the Human Phenotype Ontology (HP) ontology

Database: 
Statement: 

Display all root terms from the Human Phenotype Ontology (HP) ontology. A limit of results per page $number$ (e.g., 10)

Query: 
PREFIX owl: <http://www.w3.org/2002/07/owl#> PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#> SELECT DISTINCT ?root ?label FROM <http://bioportal.bioontology.org/ontologies/HP> WHERE { ?root a owl:Class . ?root rdfs:label ?label . OPTIONAL { ?root rdfs:subClassOf ?super } FILTER (!bound(?super)) } limit ??number