$4.$ Bonita.com sells $6800$ units of its product for $$460$ each. The selling price includes a one$-$year warranty on parts. It is expected that $3\%$ of the units will be defective and that repair costs will average $$40$ per unit. In the year of sale, warranty contracts are honored on $120$ units for a total cost of $$4800.$ What amount should Bonita.com accrue on December $31$ for estimated warranty costs?

Select answer from the options below

a$.$ $$8160$

b$.$ $$32640$

c$.$ $$4800$

d$.$ $$3360$