What event starts an action? a: habit b: trial and error c: response d: reflex e: stimulus i have more questions that i will put in comments.

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.