Question $7\mathrm{.}18-$ A circular disk weighs $50-1b$ and has a diameter of $18$ in$.$ A rod of negligible mass is used to support the disk. The system's period of torsional vibration is $1$ sec $.$ The torsional stiffness $(ft-l\frac{b}{r}ad)$ of the rod is most nearly:

A$.17$

B$.8\mathrm{.}5$

C$.3\mathrm{.}2$

D$.1\mathrm{.}8$