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 confidence
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. 99.7% confident the average concentration of PCBs in the water
supply is between 2.7 ppb and 3.3 ppb.
C. 68% confident the average concentration of PCBs in the water
supply is between 2.9 ppb and 3.1 ppb.
D. 97% confident the average concentration of PCBs in the water
supply is between 2.6 ppb and 3.8 ppb.