9.27 The increased use of wood for residential heating has caused some concern over the pollutants and irritants released into the air. In one reported study in the northwestern United States, the total suspended particulates (TSP) in nine air samples had a mean of 72 and a standard deviation of 23. Estimate the true mean TSP in a 95% confidence interval. What assumption is necessary for your answer to be valid? (See Core, Cooper, and Neulicht, J. Air Pollution Control Association, 31, no. 2,1984, pp. 138–143, for more details.)