A pharmaceutical company makes tranquilizers. It is assumed that the distribution for the length of time they last is approximately normal. Researchers in a hospital used the drug on a random sample of nine patients. The effective period of the tranquilizer for each patient (in hours) was as follows: 2.7; 2.0; 3.0; 2.5; 2.3; 2.2; 2.8; 2.1; and 2.4. a. What is the sample size? b. What is the degree of freedom? c. Construct a 90% confidence interval for the population mean length of time.