Which of these collections of subsets are partitions of {1, 2, 3, 4, 5, 6}?
a) {1, 2}, {2, 3, 4}, {4, 5, 6}
b) {1}, {2, 3, 6}, {4}, {5}
c) {2, 4, 6}, {1, 3, 5}
d) {1, 4, 5}, {2, 6}