premise: Tijuana is part of the San Diego -- Tijuana metropolitan area.
hypothesis: San Diego . It is part of the San Diego -- Tijuana conurbation , the second-largest transborder agglomeration between the US and a bordering country after Detroit -- Windsor , with a population of 4,922,723 people . The city is the seat of San Diego County and is the economic center of the region as well as the San Diego -- Tijuana metropolitan area . San Diego -- Tijuana is an international metropolitan conurbation , straddling the border of the adjacent North American coastal cities of San Diego , California , United States and Tijuana , Baja California , Mexico . Tijuana ( [ tiːˈwɑːnə ] ; [ tiˈxwana ] ) is the largest city in Baja California and on the Baja California Peninsula and center of the Tijuana metropolitan area , part of the international San Diego -- Tijuana metropolitan area .