D 
In a math competition, each participant has a unique $5$-digit registration number of the form $\overline{BBCAC}$, where $0 \leq A \textless{} B\textless{} C \leq 9$ and $B$ is the average of $A$ and $C$. What is the maximum number of participant that can join this competition? 

a. $$12$$ 
b. $$16$$ 
c. $$18$$ 
d. $$20$$ 
e. $$25$$ 