The water department checks the city water supply on a regular basis for contaminants such as xylenes. the water department takes 200 samples and estimates that the concentration of xylenes in your drinking water is 3.5 ppb (parts per billion), with a standard deviation of 0.2 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 3.3 ppb and 3.7 ppb.

b.97% confident the average concentration of pcbs in the water supply is between 3.1 ppb and 3.9 ppb.

c.99.7% confident the average concentration of pcbs in the water supply is between 2.7 ppb and 4.3 ppb.

d.68% confident the average concentration of pcbs in the water supply is between 3.3 ppb and 3.7 ppb.