From a list of 100 grade 12 students numbered 00 to 99, a sample of ten is taken. For each example below, identify what sampling method was used. Justify your choices.
a. 7, 17, 27, 37, 47, 57, 67, 77, 87, 97
b. 30, 31, 32, 33, 34, 35, 36, 37, 38, 39
c. 25, 21, 29, 28, 20, 52, 54, 50, 57, 51
d. 5, 14, 28, 38, 41, 55, 68, 70, 84,92