A student wanted to design an experiment that was for a minute that would examine the relationship between increased pollution and the frequency of asthma among high school freshmen she visited 3 different regions 1 urban 1 suburban and one rule the levels of pollution were highest in the urban region and lowest and the rural region she then surveyed high schools in eachigh schools in each region and collect the number of students with asthma in each the independent variable in this experiment is