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.
1. SI no cuidamos los bosques, no tendremos oxigeno
2. El aire estuviera mas limpio si usamos menos nuestros carros
3. Si reciclaramos, protegeriamos mucho mas el medio ambiente.
4. SI usamos el transporte publico, consumieramos menos conbustible.
5. Si cuidaramos los rios, no sufririamos por las sequias.
6. Si protegemos el planeta, todos tendremos una mejor vida.
