An environmental group is conducting a study to determine whether dolphins in a certain region are ingesting unhealthy levels of arsenic. A marine biologist classifies arsenic levels greater than 7.0 parts per million as unhealthy. The arsenic levels in a random sample of 28 dolphins in the region are measured and recorded, and the data are shown in the stemplot.
Part A: What proportion of dolphins in the sample have arsenic levels classified by the marine biologist as unhealthy? (4 points)

Part B: The mean aresenic level of the 28 dolphins in the sample is 5.26 parts per million and the standard deviation is 1.49 parts per million. Construct and interpret a 99% confidence interval for the mean arsenic level of dolphins in the region. (6 points) (10 points)