AHSME 1950 Problem 9

The area of the largest triangle that can be inscribed in a semi-circle whose radius is $r$ is: $\textbf{(A)}\ r^{2} \textbf{(B)}\ r^{3} \textbf{(C)}\ 2r^{2} \textbf{(D)}\ 2r^{3} \textbf{(E)}\ \frac{1}{2}r^{2}$