A study looked at the concentration of estrogen, a hormone that makes its way into the waterways from human waste, and fish fertility. In one study, a water sample was taken from various lakes. The data indicate that as the concentration of estrogen in the lake water goes up, the fertility level of fish in the lake goes down. The estrogen level is measured in parts per trillion (ppt) and the fertility level is recorded as the percent of eggs fertilized. a. Describe the cases in this study in a complete sentence. b. List the variables measured in this study and describe each variable as either quantitative or categorical. c. For this study, which variable is the explanatory variable? Which variable is the response variable? Briefly explain your reasoning. d. State whether this study is an experiment or an observational study? Briefly explain your reasoning.