The Concentration of Hexane (A Common Solvent) Was Measured in Units

The concentration of hexane (a common solvent) was measured in units of micrograms per liter for a simple random sample of thirteen specimens of untreated ground water taken near a municipal landfill. The sample mean was 424.2 with a sample standard deviation of 6.5. Fourteen specimens of treated ground water had an average hexane concentration of 246.0 with a standard deviation of 5.9. It is reasonable to assume that both samples come from populations that are approximately normal. Construct a 99% confidence interval for the reduction of hexane concentration after treatment.

A) (175.6, 180.8)
B) (175.1, 181.3)
C) (176.7, 179.7)
D) (170.9, 185.5)