A farmer is deciding whether or not to add fertilizer to his or her crops. If the farmer adds 1 pound of fertilizer per acre, the value of the resulting crops rises from $80 to $100 per acre. According to marginal analysis, the farmer should add fertilizer if it costs less than

$40 per pound.

$100 per pound.

$80 per pound.

$60 per pound.

$20 per pound.