AHSME 1983 Problem 23

In the adjoining figure the five circles are tangent to one another consecutively and to the lines $L_1$ and $L_2$. If the radius of the largest circle is $18$ and that of the smallest one is $8$, then the radius of the middle circle is

$\textbf{(A)} \ 12 \textbf{(B)} \ 12.5 \textbf{(C)} \ 13 \textbf{(D)} \ 13.5 \textbf{(E)} \ 14$