Debo, I think because yo goes with -o.
Las oraciones coordinadas. A través de la coordinación de oraciones formamos una oración compuesta en la que se coordinan entre sí dos ...
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.
The answer is poco i believe
