Short explanation: We use "Were" to form a question in the past simple tense when the subject is "you" and the verb is "to be".
Step-by-step solution:
- The sentence is a question in the past simple tense, indicated by "last Sunday".
- The subject is "you".
- The past tense of the verb "to be" for the subject "you" is "Were".
Answer: Were you at the zoo last Sunday?