A random variable X is being measured for a representative sample of individuals from a population. If values of X for individuals in a population are known to vary by a standard deviation of = 15, what is the minimum sample size required to ensure a 95% confidence interval for the true mean has a width of no more than 10? (a) 9 (b) 10 (c) 34.57 (d) 35 (e) 138.30 (f) 139