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Six measurements were made of the magnesium ion concentration (in parts per million, or pom) in a city's municipal water supply, with the following results. It is reasonable to assume that the population is approximately normal. Construct a 99% confidence interval for the mean magnesium ion concentration. 143 157 165 154 143 163 a. (151.4, 156.9) b. (151.6. 156.7) c. (138.5, 169.8) d. (137.0, 171.3)