Organs combine to form

which term best completes the sentence?

cells

tissues

an organism

an organ system

The answer is

Organs combine to form an organ system.

a community that experiences major changes undergoes progress

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.