The intensity of gamma radiation from a given source is $$I$$. On passing through $$36$$ mm of lead, it is reduced to $$\frac{I}{8}$$. The thickness of lead which will reduce the intensity to $$\frac{I}{2}$$ will be

A

$$6$$ mm

B

$$9$$ mm

C

$$18$$ mm

D

$$12$$ mm