What is the {chemical compound} for {has cause} of {air pollution}
What chemical compounds cause discuss pollution?
What chemical compounds cause air pollution?
SELECT DISTINCT ?i WHERE { resource:air_pollution property:has_cause ?i . ?i rdf:type ontology:chemical_compound }
select distinct ?obj where { ?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q131123> . ?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P828> . ?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> ?obj . ?obj <http://www.wikidata.org/entity/P31> <http://wikidata.dbpedia.org/resource/Q11173> } 
 select distinct ?obj where { wd:Q131123 wdt:P828 ?obj . ?obj wdt:P31 wd:Q11173 } 
carbon monoxide