a$.$ Cost of direct material part, where the cost per unit decreases with the amount of material purchased.$$ In other word, the cost of a Part A is $$10$ if $1$ to $20$ units are purchased, $$9\mathrm{.}10$ per part of $21$ to $40$ pounds are purchased, and $$8\mathrm{.}50$ per part if over $40$ parts are purchased.