Take context from the input text and decide on the accurate relationship between 'Porter Airlines' and 'TORONTO' from the options provided.
Options: chairperson, founded by, creator, headquarters location

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