In the context of the input sentence, determine the relationship between 'Safaricom' and 'Nairobi'.
Options: location of formation, founded by, developer, headquarters location

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.
headquarters location