The water department checks the city water supply on a regular basis for contaminants such as trihalomethanes (thms). the water department takes 200 samples and estimates that the concentration of thms in your drinking water is 3 ppb (parts per billion), with a standard deviation of 0.3 ppb. assuming the samples were random and unbiased how much confidencr can you have in this data? a. 95% confident the average concentration of pcbs in the water supply is between 2.4 ppb and 3.6 ppb. b. 68% confident the average concentration of pcbs in the water supply is between 2.9 ppb and 3.1 ppb..