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 - Twitter has lifted its character limit for direct messages between users, saying it would help make the private side of Twitter "more powerful and fun".

Answer: headquarters_location