The debits to Work in Process$-$Assembly Department for May, together with data concerning production, are as follows:

May $1,$ work in process:

Materials cost, $3,000$ units

Conversion costs, $3,000$ units, $66\mathrm{.}7\%$ completed

Materials added during May, $10,000$ units

Conversion costs during May

Goods finished during May, $11,500$ units

May $31$ work in process, $1,500$ units, $50\%$ completed

$$7,200$

$5,200$

$26,800$

$31,300$

All direct materials are placed in process at the beginning of the process and the first$-$in$,$ first$-$out method is used to cost inventories. The materials cost per equivalent unit for $M$

a$.$ $$3\mathrm{.}13$

b$.$ $$3\mathrm{.}85$

c$.$ $$2\mathrm{.}33$

d$.$ $$2\mathrm{.}68$