Carmen Co$.$ can further process Product J to produce Product D$.$ Product J is currently selling for $$21\mathrm{.}40$ per pound and costs $$16\mathrm{.}10$ per pound to produce. Product D would sell for $$39\mathrm{.}50$ per pound and would require an additional cost of $$8\mathrm{.}20$ per pound to produce.

What is the differential cost of producing Product D$?$

a$.$ $$9\mathrm{.}84$ per pound

b$.$ $$8\mathrm{.}20$ per pound

c$.$ $$4\mathrm{.}92$ per pound

d$.$ $$6\mathrm{.}56$ per pound