Understanding Science for 3rd Graders

A scientist was studying the effects of oil contamination on ocean seaweed. He thought that oil runoff from storm drains would keep seaweed from growing normally, so he decided to do an experiment. He filled two aquarium tanks of equal size with water and monitored the dissolved oxygen and temperature in each to be sure that they were equal. He introduced some motor oil into one tank and then measured the growth of seaweed in each tank. In the tank with no oil, the average growth was 2.57 cm. The average growth of the seaweed in the tank with oil was 2.37 cm. Based on this experiment: