What is the relationship between Twitter and SAN FRANCISCO 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.

SAN FRANCISCO (Reuters) - Critics have long lamented that Twitter Inc (TWTR.N) is difficult to use and has had trouble defining itself, which is why it has not expanded its audience.

Answer: headquarters_location