what flights do you have in the morning of september twentieth on united airlines from pittsburgh to san francisco and a stopover in denver
O O O O O O O B-depart_time.period_of_day O B-depart_date.month_name B-depart_date.day_number O B-airline_name I-airline_name O B-fromloc.city_name O B-toloc.city_name I-toloc.city_name O O O O B-stoploc.city_name