Utilize the input text as a context reference, choose the right relationship between Twitter and SAN FRANCISCO from the options. Options: 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