instruction: what airlines fly between BOSTON and SAN FRANCISCO and stop in DENVER
response: SELECT DISTINCT airline.airline_code FROM airline, airport_service AS AIRPORT_SERVICE_0, airport_service AS AIRPORT_SERVICE_1, airport_service AS AIRPORT_SERVICE_2, city AS CITY_0, city AS CITY_1, city AS CITY_2, flight, flight_stop WHERE (CITY_1.city_code = AIRPORT_SERVICE_1.city_code AND CITY_1.city_name = 'SAN FRANCISCO' AND CITY_2.city_code = AIRPORT_SERVICE_2.city_code AND CITY_2.city_name = 'DENVER' AND flight_stop.stop_airport = AIRPORT_SERVICE_2.airport_code AND flight.flight_id = flight_stop.flight_id AND flight.to_airport = AIRPORT_SERVICE_1.airport_code) AND CITY_0.city_code = AIRPORT_SERVICE_0.city_code AND CITY_0.city_name = 'BOSTON' AND flight.airline_code = airline.airline_code AND flight.from_airport = AIRPORT_SERVICE_0.airport_code