The cost of a computer system increases with increased processor speeds. The cost $C$ of a system as a function of processor speed is estimated as $C=13S^2-9S+1800,$ where $S$ is the processor speed in $MHz.$ Find the processor speed for which cost is at a minimum.

A$.0\mathrm{.}5MHz$

B$.2\mathrm{.}8MHz$

C$.0\mathrm{.}3MHz$

D$.6\mathrm{.}9MHz$