Harry, a wheat farmer, is deciding whether or not to add fertilizer to his crops. If he adds 1 pound of fertilizer per acre, the value of the resulting crops rises from $80 to $100 per acre. According to marginal analysis, Harry should add fertilizer if it costs less than:
A) $100 per pound
B) $12.50 per pound
C) $80 per pound
D) $20 per pound