Which are the correct spellings for 21 and 23? veinti i uno, veintitres
veintiuno, veintitrés
veintiuno, véintitres
veinti y uno, veintitrés

I think it’s B
Veintiuno, veintitrés

Ithink its b) built underground reservoirs

For this question, it will be important to use the words "mejor" (better), "lo mejor" (the best), and "lo peor" (the worst). while "mejor" is comparative (you are comparing one thing to something else), "lo mejor" and "lo peor" are superlative (there is nothing else better or worse than what these words are describing - there is no comparison).  you could say something like: la música rap es mejor que la música clásica - que es lo peor - pero la música rock es lo mejor de todas las formas de la música (rap music is better than classical music - which is the worst - but rock music is the best out of all the forms of music).