7. Sarah designed an experiment to find out which mouthwash was most effective against some bacteria. She cut out four different circles from a paper towel and soaked each circle in a different mouthwash. She put the
circles on a nutrient agar-coated Petri dish that was covered with bacteria commonly found in the mouth. She
then incubated the plate for 24 hours. The picture shows the results of this test,
> What should Sarah do to improve her experiment?