Is the sentence grammatically correct or not?
1. correct or incorrect
5. correct or incorrect
6. correct or incorrect

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).

Right your are correct

The correct answer is
1. black death

a deadly plague that swept through europe between 1347 and 1351, it killed one third of european population, but the left food supply give chances to survivors for advancement.

and

2. smallpox

an eradicated virus that used to be contagious, disfiguring, and often deadly.