AHSME 1961 Problem 14

A rhombus is given with one diagonal twice the length of the other diagonal. Express the side of the rhombus is terms of $K$, where $K$ is the area of the rhombus in square inches. $\textbf{(A)}\ \sqrt{K} \textbf{(B)}\ \frac{1}{2}\sqrt{2K} \textbf{(C)}\ \frac{1}{3}\sqrt{3K} \textbf{(D)}\ \frac{1}{4}\sqrt{4K} \textbf{(E)}\ \text{None of these are correct}$