Biochemical oxygen demand (BOD) measures organic pollutants in water by measuring the amount of oxygen consumed by microorganisms that break down these compounds. BOD is hard to measure accurately. Total organic carbon (TOC) is easy to measure, so it is common to measure TOC and use regression to predict BOD. A typical regression equation for water entering a municipal treatment plant is BOD= -55.42 + 1.502 TOC Both BOD and TOC are measured in milligrams per liter of water. (a) What does the slope of this line say about the relationship between BOD and TOC? O TOC rises (falls) by 1.502 mg/l for every 1 mg/l increase (decrease) in BOD OBOD rises (falls) by 1.502 mg/l for every 1 mg/l increase (decrease) in TOC O BOD rises (falls) by 55.42 mg/l for every 1 mg/l increase (decrease) in TOC O TOC rises (falls) by 1.502 mg/l for every 55.42 mg/l increase (decrease) in BOD (b) What is the predicted BOD when TOC = 0? Values of BOD less than 0 are impossible. Why do you think the prediction gives an impossible value? O This arises from extrapolation; the data used to find this regression formula must not have included values of TOC near 0. O The regression equation incorrect; correct regression equation would never provide impossible values. O There must be lurking variables; these factors have created a regression equation that allows for impossible values.