Los griegos fueron grandes navegantes y costruyeron unos barcos especiales para la lucha, que denominaban birremes o trirremes segun las filas de remeros que tenian.
The answer is (usted) because you dont have a personal relationship with this person.
ustedes means you guys
vosotros means all of us
tu is used for someone whom you are close with like a family member or someone whom you have know for a while.
