The following 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 below are available for purchase.

CPU GHz	Total Benefit	Marginal Benefit	Total Cost	Marginal Costs
2.0	$1,000		$900
2.5	$1,400			$100
3.0		$300	$1,200
3.5	$1,900		$1,500
4.0	$2,000			$400

The marginal cost of upgrading from a 2.5GHz to 3.0GHz computer is:

$400

$300

$200

$100