Life tests performed on a sample of 13 batteries of a new model indicated: (1) an average life of 75 months, and (2) a standard deviation of 5 months. Other battery models, produced by similar processes, have normally distributed life spans. The 90% confidence interval for the population mean life of the new model is _________.

Select one:

a.66.09 to 83.91

b.72.72 to 77.28

c.66.78 to 83.23

d.72.53 to 77.47