Measurements of water quality were taken a river downstream from an abandoned chemical dumpsite. concentrations of a certain chemical were were obtained from 9 measurements taken at the surface of the water, 9 measurements taken at mid-depth of the water. and 9 measurements taken at the bottom of the water. what type of study was conducted, and what is the response variable of the study?
(a) an experiment was conducted, response variable is the connection of the chemical
(b) an experiment was conducted, response variable is the depth of the water
(c) a census was conducted, and the response variables is the depth of the water
(d) an observational study was conducted, and the response variable is the depth of the water