a bottling plant fills one-gallon jugs with milk. the labels on the jugs state that the volume of milk they contain is 128 fluid ounces (fl. oz.). federal law mandates that the jugs must contain no less than the stated volume. the actual volume of milk in the jugs is normally distributed with mean μ=129 fl. oz. and standard deviation σ=0.8 fl. oz. plant workers take a simple random sample (srs) of 8 jugs, measure the volume of milk in each jug, and calculate the sample mean. what is the shape of the distribution of sample means?