The accompanying 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 in the table are available for purchase.

$\backslash$table$[[$CPU GHzTotal BenefitMarginal BenefitTotal CostMargina,C$,,],[2\mathrm{.}0,$$$1,000,$$$900,,],[2\mathrm{.}5,$$$1,400,,$$$100,],[3\mathrm{.}0,,$$$300,$$$1,200,],[3\mathrm{.}5,$$$1,900,,$$$1,500,],[4\mathrm{.}0,$$$2,000,,$$$400,]]$

The marginal benefit of upgrading from a $2\mathrm{.}0GHz$ computer to a $2\mathrm{.}5GHz$ computer is

Select one:

a$.$ $$1,000.$

b$.$ $$1,400.$

c$.$ $$400.$

d$.$ $$100.$