What is the relationship between 'Porter Airlines' and 'TORONTO' in the context of the input sentence.
Options: developer, founded by, location of formation, platform

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.
location of formation