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 - Russian government-backed hackers who penetrated high-profile U.S. government and defense industry computers this year used a method combining Twitter with data hidden in seemingly benign photographs, according to experts studying the campaign.

Answer: headquarters_location