An ecological group suspects dolphins are consuming dangerous levels of arsenic in a region of the world. Dangerous levels of arsenic are classified when they reach beyond 7.0 parts per million. A random sample of 28 dolphins in the region were tested and the data is shown in the stemplot. Arsenic levels:
2|4
2|
3|2, 3
3|6, 7, 9
4|0, 1, 3
4|5, 7, 7
5|0, 1, 1, 4, 4
5|7, 8
6|3, 4
6|6
7|0, 1, 3
7|5, 6, 7

Key: 2|4 = 2.4 parts per million

Part A: What proportion of the dolphins in the sample have dangerous levels of arsenic in their systems?

Part B: Construct and interpret a 99% confidence interval for the mean arsenic levels of the dolphins if the sample of 28 dolphins has a mean arsenic level of 5.26 parts per million and standard deviation of 1.49 parts per million.​