In the context of the input sentence, determine the relationship between 'medium' and 'San Francisco'.
Options: manufacturer, headquarters location, platform, location of formation

SAN FRANCISCO, Sept 25 Video gamers who live-stream their matches online and the throngs of fans who tune in daily to watch them swarmed a first-of-its kind convention in San Francisco on Friday, as interest swells in the medium of online video game broadcasting.
headquarters location