Bridges over which rivers are made of Wrought Iron?
SELECT DISTINCT ?uri WHERE { ?x <http://dbpedia.org/property/material> <http://dbpedia.org/resource/Wrought_iron> . ?x <http://dbpedia.org/ontology/river> ?uri  . ?x <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/Bridge>}
Wrought_iron