What is the relationship between Porter Airlines and TORONTO in the context of the input sentence. Choose an answer from: 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