"The population of Mexico City is very big."
Translation:La población de la Ciudad de México es muy grande.
Duolingo Forum? Where does it say DF? What trick ❓ Has it been removed now or something? So many questions, but I am baffled here.
The second says "la cultura" and they are talking about "la población/ the population", they are different things.
La Ciudad de México is called just México in Mexico. Duo should have known that.