Studies of sedimentary rocks and fossils have helped scientists understand the history of Earth. How is geologic time organized on the geologic time scale?

millennia and then smaller periods based on geologic and biologic events that occurred during those times

eras and then smaller periods based on geologic and biologic events that occurred during those times

eras and then smaller millennia based on geologic and biologic events that occurred during those times

periods and then smaller eras based on geologic and biologic events that occurred during those times