Which of the following shows the union of the sets? {4, 8, 12, 16, 20, 24, 28} {2, 4, 6, 8, 10, 12} A. {4, 8, 12} B. {4, 8, 12, 16} C. {6, 8, 10, 12} D. {2, 4, 6, 8, 10, 12, 16, 20, 24, 28}