Which of the following would best improve the validity of the experiment? Eliminating tank 1, because the DO stayed the same Eliminating tank 1, because the DO stayed the same A Observing the results of adding organic waste to tanks containing salt water Observing the results of adding organic waste to tanks containing salt water B Repeating the experiment several times and comparing the results Repeating the experiment several times and comparing the results C Increasing the amounts of green algae in each of the tanks, with tank 5 having the greatest amount Increasing the amounts of green algae in each of the tanks, with tank 5 having the greatest amount D Adding different types of waste to each tank and then checking the results