Utilize the input text as a context reference, choose the right relationship between Safaricom and Nairobi 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.

With over 12 million subscribers accessing the internet through its network, Safaricom pioneered its LTE Advanced (4G) proposition in December 2014, launching the most advanced version of the 4G system for mobile networks in Nairobi and Mombasa.

Answer: headquarters_location