What is the relationship between Boris Johnson and Mayor of London in the context of the input sentence. Choose an answer from: 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