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.

Pulse at Twitter Tech hiring in the San Francisco Bay Area is especially challenging because so many new companies are competing for workers there.

Answer: headquarters_location