For Concord Corporation at a sales volume of $4000$ units, sales revenue is $$83000,$ variable costs total $$42000,$ and fixed expenses are $$21000.$ What is the unit contribution margin?

$$15\mathrm{.}50$

$$10\mathrm{.}50$

$$10\mathrm{.}25$

$$5\mathrm{.}25$