Utilize the input text as a context reference, choose the right relationship between 'Porter Airlines' and 'TORONTO' from the options.
Options: headquarters location, creator, founded by, chairperson

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.
headquarters location