What is the degree of drawing equal to?

Question

         In drawing operation if D_i = initial diameter and D₀ = outgoing diameter then what is thedegree of drawing equal to?

(a) $\frac{D_{i}-D_{o}}{D_{i}}$

(b) $\frac{D_{o}-D_{i}}{D_{o}}$

(c) $\frac{D^{2}_{i}-D^{2}_{o}}{D^{2}_{i}}$

(d) $\sqrt{\frac{D_{i}^{2}-D_{o}^{2}}{D^{2}_{i}}}$