Basel uses the high$-$low method of estimating costs. Basel had total costs of $$50,000$ at its lowest level of activity, when $5,000$ units were sold. When, at its highest level of activity, sales equaled $12,000$ units, and total costs were $$78,000.$

Basel would estimate variable cost per unit as

a$.$ $$10$

b$.$ $$6\mathrm{.}50$

c$.$ $$7\mathrm{.}53$

d$.$ $$4$