AMC 12 2016a Problem 6

A triangular array of $2016$ coins has $1$ coin in the first row, $2$ coins in the second row, $3$ coins in the third row, and so on up to $N$ coins in the $N$th row. What is the sum of the digits of $N$ ?

\textbf{(A)}\ 6 \textbf{(B)}\ 7 \textbf{(C)}\ 8 \textbf{(D)}\ 9 \textbf{(E)}\ 10