2000 Cayley Problem 5

The first four triangular numbers 1, 3, 6 and 10 are illustrated in the diagram. What is the tenth triangular number?

\textbf{(A)}\ 55\quad \textbf{(B)}\ 45\quad \textbf{(C)}\ 66\quad \textbf{(D)}\ 78\quad \textbf{(E)}\ 50

If there are no answer choices shown, enter a numerical answer.

Full credit to this problem is given to the CEMC, you may view all Cayley contests here.