AHSME 1980 Problem 19

Let $C_1, C_2$ and $C_3$ be three parallel chords of a circle on the same side of the center. The distance between $C_1$ and $C_2$ is the same as the distance between $C_2$ and $C_3$. The lengths of the chords are $20, 16$, and $8$. The radius of the circle is $\text{(A)} \ 12 \text{(B)} \ 4\sqrt{7} \text{(C)} \ \frac{5\sqrt{65}}{3} \text{(D)}\ \frac{5\sqrt{22}}{2} \text{(E)}\ \text{not uniquely determined}$