If the perimeter of a semi-circular protractor is 108 cm, find the diameter of the protractor.

Question

$\begin{array}{1 1} (a)\; \text{42 cm} \\ (b) \; \text{21 cm} \\ (c)\; \text{$\frac{21}{2}$ cm} \\ (d) \text{none of these} \end{array}$