list the number of flights leaving boston for dallas fort worth before 9 am in the morning
O O O O O O B-fromloc.city_name O B-toloc.city_name I-toloc.city_name I-toloc.city_name B-depart_time.time_relative B-depart_time.time I-depart_time.time O O B-depart_time.period_of_day