1. Which of the following is incorrectly written set? Why? a. S = {1,2,3,4} b. S= {a, b, c, d} c. S= {d, c, b, a} d. S= {1,3,90, a, Hello} e. S= {1, 2, 3, 1, 2, 3} f. S= {a, b, c, d, c, b, a}