3. A sunk XXXX XX a retrospective (past) XXXX XXXX XXX already XXXX incurred XXX XXXXXX XX recovered. XXXX XXXXX XXX sometimes contrasted XXXX prospective XXXXX while XX marginal cost XX the change in the total XXXX that XXXXXX XXXX XXX XXXXXXXX XXXXXXXX has an XXXXXXXXX by unit. XXXX is, it XX XXX XXXX of XXXXXXXXX XXX XXXX XXXX of a good. XX XXXXXXX XXXXX, marginal cost at XXXX XXXXX XX production XXXXXXXX XXX additional costs XXXXXXXX XX XXXXXXX the XXXX unit.