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.
Refer to the input text as context and select the correct relationship between 'Porter Airlines' and 'TORONTO' from the available options.
Options: founded by, platform, location of formation, developer