What is the {flammable gas} for {has cause} of {air pollution}
Air pollution is caused by which flammable gas?
What flammable gas is a big cause for air pollution ?
SELECT DISTINCT ?n WHERE { ?n rdf:type ontology:flammable_gas . resource:air_pollution property:has_cause ?n }
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/Q10913792> } 
 select distinct ?obj where { wd:Q131123 wdt:P828 ?obj . ?obj wdt:P31 wd:Q10913792 } 
carbon monoxide