Question: who all play for of the american football players at  Quarterback position?
SPARQL:
SELECT DISTINCT ?uri WHERE { ?x <http://dbpedia.org/property/position> <http://dbpedia.org/resource/Quarterback> . ?x <http://dbpedia.org/ontology/team> ?uri  . ?x <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/AmericanFootballPlayer>}