When considering the work value of chlorine, which of the following values provides the GREATEST benefit to the chemical performance?

a. free chlorine 0.2 ppm, total chlorine 0.8, pH 7.8, cyanuric acid 0 ppm b. free chlorine 0.5 ppm, total chlorine 1.1, pH 8.2, cyanuric acid 50 ppm c. free chlorine 1.5 ppm, total chlorine 1.5, pH 7.3, cyanuric acid 0 ppm d. free chlorine 3.0 ppm, total chlorine 4.2, pH 8.0, cyanuric acid 90 ppm