Question about Spanish book activity

Question

There is an activity in a Spanish textbook that I am confused about.
Example from textbook: Tú ¿Cual es tu clase más divertida? Compañero(a) Economía es la clase más divertida.

I previously asked a question about how I answered one of the questions to check for accuracy. Here is the corrected answer to the one that I did.

¿Cuál es tu clase favorita? La de teatro es mi clase favorita.

Based on the example in the textbook, I answered "Teatro es la clase favorita." I think it makes more sense to say "mi clase" vs. "la clase".

So my questions are 1 - why would the above example answer in the textbook not be "La economía es mi clase más divertida."
And 2 - why is it "la de teatro" and not just "la teatro"?

Any help would be greatly appreciated!

Answer 1

Could both be correct? They're two ways to skin a cat. With respect to la clase de treatro, you have to say la de teatro because you're omitting clase, right. La teatro means something different and the gender is wrong.