1) Where is Mary? - Mary is in Mexico City.( So I am in Mexico City.)
2) What language does she learn? - She learns Spanish. (I learn Spanish.)
3) Does she work hard? - Yes, she does. She works very hard.(I work very hard.)
4) Is Mexico City larger than Cleveland? - Yes it is. Mexico City is much larger than Cleveland. (Mexico City is much larger than Cleveland,)
5) Is it more interesting? - Yes, it is more interesting.
6) Is Mexico City the biggest city in the world? - по тексту она считает его самым лучшим городом в мире (I think it is the best city in the world..), а в вопросе спрашивается Мексико-сити самый большой (the biggest city) город в мире? Может в тексте ошибка или в вопросе?
7) Is there too much traffic and smog in the city? - Yes, it is. There's too much traffic and smog. (But there's too much traffic and smog,)
8) Are there many restaurants with American food there? - There are not enough restaurants with American food. (..and there are not enough restaurants with American food,)
9) Are Mary's teachers very nice? - Yes, all of Mary's teachers are very nice. (All of my teachers are very nice.)
10) Do they speak English to their students? - No, they are not. None of them speak English to the students. (None of them speak English to us..)
В скобках - цитаты из текста для подтверждения ответа.