instruction: which of the flights from PITTSBURGH to BALTIMORE on 7 25 1991 carries the smallest number of passengers
response: SELECT DISTINCT FLIGHT_0.flight_id FROM aircraft, airport_service AS AIRPORT_SERVICE_0, airport_service AS AIRPORT_SERVICE_1, airport_service AS AIRPORT_SERVICE_2, airport_service AS AIRPORT_SERVICE_3, city AS CITY_0, city AS CITY_1, city AS CITY_2, city AS CITY_3, date_day, days, equipment_sequence AS EQUIPMENT_SEQUENCE_0, equipment_sequence AS EQUIPMENT_SEQUENCE_1, flight AS FLIGHT_0, flight AS FLIGHT_1 WHERE ((aircraft.capacity = (SELECT MIN(AIRCRAFTalias1.capacity) FROM aircraft AS AIRCRAFTalias1, airport_service AS AIRPORT_SERVICEalias4, airport_service AS AIRPORT_SERVICEalias5, city AS CITYalias4, city AS CITYalias5, equipment_sequence AS EQUIPMENT_SEQUENCEalias2, flight AS FLIGHTalias2 WHERE CITYalias4.city_code = AIRPORT_SERVICEalias4.city_code AND CITYalias4.city_name = 'PITTSBURGH' AND CITYalias5.city_code = AIRPORT_SERVICEalias5.city_code AND CITYalias5.city_name = 'BALTIMORE' AND EQUIPMENT_SEQUENCEalias2.aircraft_code = AIRCRAFTalias1.aircraft_code AND FLIGHTalias2.aircraft_code_sequence = EQUIPMENT_SEQUENCEalias2.aircraft_code_sequence AND FLIGHTalias2.from_airport = AIRPORT_SERVICEalias4.airport_code AND FLIGHTalias2.to_airport = AIRPORT_SERVICEalias5.airport_code) AND CITY_2.city_code = AIRPORT_SERVICE_2.city_code AND CITY_2.city_name = 'PITTSBURGH' AND CITY_3.city_code = AIRPORT_SERVICE_3.city_code AND CITY_3.city_name = 'BALTIMORE' AND date_day.day_number = 25 AND date_day.month_number = 7 AND date_day.year = 1991 AND days.day_name = date_day.day_name AND EQUIPMENT_SEQUENCE_0.aircraft_code = aircraft.aircraft_code AND EQUIPMENT_SEQUENCE_1.aircraft_code = aircraft.aircraft_code AND FLIGHT_0.aircraft_code_sequence = EQUIPMENT_SEQUENCE_0.aircraft_code_sequence AND FLIGHT_0.flight_days = days.days_code AND FLIGHT_1.aircraft_code_sequence = EQUIPMENT_SEQUENCE_1.aircraft_code_sequence AND FLIGHT_1.from_airport = AIRPORT_SERVICE_2.airport_code AND FLIGHT_1.to_airport = AIRPORT_SERVICE_3.airport_code) AND CITY_1.city_code = AIRPORT_SERVICE_1.city_code AND CITY_1.city_name = 'BALTIMORE' AND FLIGHT_0.to_airport = AIRPORT_SERVICE_1.airport_code) AND CITY_0.city_code = AIRPORT_SERVICE_0.city_code AND CITY_0.city_name = 'PITTSBURGH' AND FLIGHT_0.from_airport = AIRPORT_SERVICE_0.airport_code