Identify the mode for each of the following sets of data:

a. 6, 2, 7, 6, 6, 4

b. 13, 19, 12, 13, 7, 13, 20, 13, 15

c. 4, 2, 5, 1, 2, 2, 5, 5, 3, 1, 5, 2, 6, 3

d. 40, 10, 35, 30, 10, 25, 5, 10, 15, 10, 5, 35, 10, 40, 20

e. 1, 4, 2, 4, 4, 1, 5, 1, 4, 2, 3, 1, 4, 2, 3, 5, 1