Why were cenotes important to the Maya? They were valuable trade items.
They were fertile areas for farming.
They were used to perform sacrifices.
They were the main source of drinking water.

What document are you referring to

Ifound the exercise on the internet. attached is the list mentioned above.in order for a compound to be considered optically active it can't have any axis or plan of symmetry. the molecule  shouldn't also  be superimposable on its mirror image.with such considerations, the compounds that are optically active are: -third compound on the first image is optically active; -second and third compound from the second image are optically actives.