2011 Cayley Problem 16

A gumball machine that randomly dispenses one gumball at a time contains 13 red, 5 blue, 1 white, and 9 green gumballs. What is the least number of gumballs that Wally must buy to *guarantee* that he receives 3 gumballs of the same colour?

\textbf{(A)}\ 6\quad \textbf{(B)}\ 9\quad \textbf{(C)}\ 4\quad \textbf{(D)}\ 7\quad \textbf{(E)}\ 8

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.