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

Multiple Choice

The concentration of hexane (a common solvent) was measured in units of micrograms per liter for a simple random sample of twelve specimens of untreated ground water taken near a municipal Landfill. The sample mean was 720.2 with a sample standard deviation of 8.8. Eleven specimens of Treated ground water had an average hexane concentration of 695.1 with a standard deviation of 9.1.
It is reasonable to assume that both samples come from populations that are approximately normal. Construct A 90% confidence interval for the reduction of hexane concentration after treatment.

A) (23.6, 26.6)
B) (18.3, 31.9)
C) (21.8, 28.4)
D) (22.2, 28.0)