The Following Table Shows the Relationship Between the Speed of a Computer's

The following table shows the relationship between the speed of a computer's CPU and its benefits and costs. Assume that all other features of the computer are the same (that is, CPU speed is the only source of variation) , and only the CPU speeds listed below are available for purchase. $$\begin{array} { | c | c | c | c | c | } \hline \begin{array} { c } \text { CPU } \\\mathrm { GHz }\end{array} & \begin{array} { c } \text { Total } \\\text { Benefit }\end{array} & \begin{array} { c } \text { Marginal } \\\text { Benefit }\end{array} & \begin{array} { c } \text { Total } \\\text { Cost }\end{array} & \begin{array} { c } \text { Marginal } \\\text { Cost }\end{array} \\\hline 2.0 & \$ 1,000 & & \$ 900 & \\\hline 2.5 & \$ 1,400 & & & \$ 100 \\\hline 3.0 & & \$ 300 & \$ 1,200 & \\\hline 3.5 & \$ 1,900 & & \$ 1,500 & \\\hline 4.0 & \$ 2,000 & & & \$ 400 \\\hline\end{array}$$ The marginal cost of upgrading from a 2.5GHz to 3.0GHz computer is:

A) $400.
B) $300.
C) $200.
D) $100.