Utilize the input text as a context reference, choose the right relationship between Boris Johnson and Mayor of London 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.

London is a key test-bed for the UK hydrogen economy and also on Tuesday, the city took delivery of a dozen new zero-emission vehicles from Toyota , powered by hydrogen fuel cells made by UK-based ITM Power (LSE: ITM.L - news ) , while Mayor of London Boris Johnson also timed a visit to the Japanese headquarters of the car manufacturer on the same day.

Answer: position_held