Study the pattern below. The $$100^{}\rm {th}$$ letter is  .  $$A A B B C A A B B C A A B B C A A \cdots$$ 
Overseas Competition->Knowledge Point->Word Problem Modules->Periodic Problems->Periodic Problems of Basic Permutation
$$100$$~$\div$ $$5$$ = $$20$$ $$R0$$. 