Most of the rocks we study were formed in what era?
a. cenozoic
b. paleozoic
c. precambrian
d. mesozoic

Paleozoic

Explanation:

Not as many rocks existed from the precambrian era

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.