Local pharmacies have special handling costs of $$0\mathrm{.}10$ per case, and sales commissions are paid to agents costing $$0\mathrm{.}25$ per case. Bad debt expense averages $10\%$ of sales.

What customer type has the least total cost per case?

a$.$ gas station chains

b$.$ local pharmacies

c$.$ drugstore chains

d$.$ supermarket chains