which molecule is most common in the human body?

I’m doing some practice exam questions for biology, and i’m not sure how to do these 3 parts to the question.

Adigestive system that is a series of tubes beginning at the mouth and ending at the anus is a digestive system.

Agenus is a taxonomic grouping that identifies a collection of species. when a scientist discovers a new species, how is this species assigned to a genus