Question: Air Canada Flight 797 started out at what largest hub for American Airlines?

Context paragraphs:
Air Canada destinations: Air Canada is the largest airline and flag carrier of Canada.  Founded in 1936 as Trans-Canada Airlines, it provides scheduled services to 194 destinations on five continents.  Its largest hub is Toronto Pearson International Airport, followed by Montreal-Pierre Elliott Trudeau International Airport, Vancouver International Airport and Calgary International Airport.  Air Canada is the world's 10th largest passenger airline by fleet size, and the airline is a founding member of Star Alliance.  In 2014, Air Canada together with its Air Canada Express regional partners carried over 38 million passengers.  Between them, they operate on average more than 1,500 scheduled flights daily.

Air Canada Flight 797: Air Canada Flight 797 was a scheduled trans-border flight that flew from Dallas/Fort Worth International Airport to Montréal–Dorval International Airport, with an intermediate stop at Toronto Pearson International Airport.  On 2 June 1983, the McDonnell Douglas DC-9-32 operating the service developed an in-flight fire behind the washroom that spread between the outer skin and the inner decor panels, filling the plane with toxic smoke.  The spreading fire also burned through crucial electrical cables that knocked out most of the instrumentation in the cockpit, forcing the plane to divert to Cincinnati/Northern Kentucky International Airport.  Ninety seconds after the plane landed and the doors were opened, the heat of the fire and fresh oxygen from the open exit doors created flashover conditions, and the plane's interior immediately became engulfed in flames, killing 23 passengers who had yet to evacuate the aircraft.  Blood tests showed some of the passengers had inhaled lethal amounts of toxins from the in-flight fire, likely meaning they were already dead in their seats before the plane touched down.

Nathan Rogers: Nathan Rogers (born July 16, 1979 in Hamilton, Ontario) is a Canadian folk musician/songwriter.  He is the son of Stan and Ariel Rogers.  His father, a folk musician and songwriter, died in a fire aboard Air Canada Flight 797 on June 2, 1983.

Air Canada Rouge: Air Canada Rouge, stylized Air Canada rouge, is the vacation and leisure airline subsidiary of Air Canada.  Air Canada Rouge is fully integrated into the Air Canada mainline and Air Canada Express networks, flights are sold with AC flight numbers but are listed as "operated by Air Canada Rouge" (similar to regional flights operated under the Air Canada Express banner).  "Rouge" means "red" in French.

Air Canada: Air Canada () is the flag carrier and largest airline of Canada.  The airline, founded in 1937, provides scheduled and charter air transport for passengers and cargo to 182 destinations worldwide.  It is the world's eighth-largest passenger airline by fleet size, and is a founding member of the Star Alliance.  Air Canada's corporate headquarters are located in Montreal, Quebec, while its largest hub is at Toronto Pearson International Airport.  Air Canada had passenger revenues of CA$13.8 billion in 2015.  The airline's regional service is Air Canada Express.

Air Canada Flight 646: Air Canada Flight 646 was a flight from Toronto's Lester B. Pearson International Airport to Fredericton, New Brunswick, operated by Air Canada.  On December 16, 1997, at 23:48 local time, the Canadair CRJ-100ER(CL-65) jet crashed after a failed go-around attempt in Fredericton.  All passengers and crew survived, despite a 20-minute emergency response time and inadequate emergency training of the flight crew.

Air Canada Flight 189: Air Canada Flight 189 was an Air Canada flight from Ottawa to Vancouver via Toronto and Winnipeg.  On June 26, 1978, it crashed on takeoff in Toronto killing two passengers.

Caterpillar 797B: The Caterpillar 797B is an off-highway, ultra class, two-axle, mechanical powertrain haul truck developed and manufactured by Caterpillar Inc. in the United States specifically for high production mining and heavy-duty construction applications world-wide.  The 797B is the second generation of the 797 series.  The 797B was introduced to the public in spring of 2002 and first entered service with a customer in October, 2002.  The 797B remained in production until December, 2009 when Caterpillar started full production of the 797B's successor, the 797F.  While it was in production, the 797B was Caterpillar's largest, highest payload capacity haul truck model.

Air Canada Flight 621: The Air Canada Flight 621 crash near Toronto Pearson International Airport, took place on July 5, 1970, when an Air Canada Douglas DC-8, registered as CF-TIW, was attempting to land.  It was flying on a Montreal–Toronto–Los Angeles route.

Dallas/Fort Worth International Airport: Dallas/Fort Worth International Airport (IATA: DFW, ICAO: KDFW, FAA LID: DFW) is the primary international airport serving the Dallas–Fort Worth metroplex area in the U.S. state of Texas.  It is the largest hub for American Airlines, which is headquartered near the airport.  2016 was a record year for DFW, as the airport served 65,670,697 passengers.

Supporting paragraphs: Air Canada Flight 797 Dallas/Fort Worth International Airport
Answer: Dallas/Fort Worth International Airport