Answer: tú eres responsable.
You are responsible.
Explanation: The subject in this sentence is "tú", that is the second-person in the singular, so the verb must be conjugated in this form. Tú eres.
1. b. you use queria because if you say "quise" it means you stopped wanting to learn ballet sometime in the past.
2. b. you use conoci' because you've already met the professor in the past
Answer:
a juan y a pablo les gusta poner la comida en la mesa