AMC 12 2005a Problem 3

A rectangle with a diagonal of length $x$ is twice as long as it is wide. What is the area of the rectangle?

$\textbf{(A) } \frac{1}{4}x^2 \textbf{(B) } \frac{2}{5}x^2 \textbf{(C) } \frac{1}{2}x^2 \textbf{(D) } x^2 \textbf{(E) } \frac{3}{2}x^2$