Solve Each Problem $C$ Of a System as a Function of Processor Speed Is

Multiple Choice

Solve each problem.
-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 $C=12 S^{2}-9 S+1000$ , where $S$ is the processor speed in MHz. Find the processor speed for which cost is at a minimum.

A) $0.4 \mathrm{MHz}$
B) $0.5 \mathrm{MHz}$
C) $3 \mathrm{MHz}$
D) $7.5 \mathrm{MHz}$