The Children's Ditty Which Says "Bill and Susie Sitting in a Tree

The children's ditty which says "Bill and Susie sitting in a tree, K-I-S-S-I-N-G, first comes love, then comes marriage, then comes Susie pushing the baby carriage" is illustrative of

A) transitions.
B) event and stage sequences in normative timing.
C) developmental task.
D) variation in family careers.
E) all of the above.