There was a change in the environment between
generations 10 and 30. the change likely
caused a genetic mutation
eliminated a recessive trait
favored trait a
favored trait b

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.