A student investigated the effectiveness of four different mouthwashes in destroying bacteria. He innoculated the nutrient agar in four petri dishes with bacteria. Each of four paper disks, 1 centimeter in diameter, was soaked in a different mouthwash sample, and placed on a different agar surface. Sterile procedures were used throughout the experiment. Each petri dish was place in an incubator at a temperature of 37 degrees Celsius for a 24-hour period. The diagram represents the sequence of events in this investigation. The shaded areas in the petri dishes represent regions of bacterial growth.
Which petri dish contains the most effective mouthwash and why?
Select one of the following: