Utilize the input text as a context reference, choose the right relationship between Porter Airlines and TORONTO from the options. Options: product/material produced, manufacturer, distributed by, industry, position held, original broadcaster, owned by, founded by, distribution format, headquarters location, stock exchange, currency, parent organization, chief executive officer, director/manager, owner of, operator, member of, employer, chairperson, platform, subsidiary, legal form, publisher, developer, brand, business division, location of formation, creator.

TORONTO Aug 20 (Reuters) - Regulators threatened to ground Canada's Porter Airlines over safety problems in 2008, according to documents reviewed by Reuters, but the matter was kept secret for years - a sign, some critics say, of how little the public is told about the safety of Canadian airlines.

Answer: headquarters_location