AMC 8 1995 Problem 6

Figures $I$, $II$, and $III$ are squares. The perimeter of $I$ is $12$ and the perimeter of $II$ is $24$. The perimeter of $III$ is

$\text{(A)}\ 9 \text{(B)}\ 18 \text{(C)}\ 36 \text{(D)}\ 72 \text{(D)}\ 81$