Students want to conduct an experiment to determine how road construction impacts the amount of PM in the air. They spread vaseline onto the bottom of six petri dishes and placed them at various distances from a road construction site and an existing road. The first three dishes were placed 50, 100, and 200 yards from the road construction site. The second group of dishes were placed 50, 100, and 200 yards from an active road (not under construction). Identify the control group used in this experiment.

Identify the likely dependent variable the students are measuring and describe how they could measure this variable.