How many rivers are crossed by different Box Girder bridges?
SELECT DISTINCT COUNT(?uri) WHERE { ?x <http://dbpedia.org/property/design> <http://dbpedia.org/resource/Box_girder_bridge> . ?x <http://dbpedia.org/property/crosses> ?uri  . ?uri <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/River>}
Box_girder_bridge