Each participant in a certain study was assigned a sequence of 3 different letters from the
set {A, B, C, D, E, F, G, H}. If no sequence was assigned to more than one participant
and if 36 of the possible sequences were not assigned, what was the number of
participants in the study? (Note, for example, that the sequence A, B, C is different from
the sequence C, B, A.)
答案：C