Although arid and semi-arid regions of the world make up 40 percent of the earth’s surface, what percent of the world’s freshwater runoff do they receive?

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.

i think the answer is a.keratin is the pigment responsible for skin color.

. a balance and a graduated cylinder.