3. A XXXX cost is a retrospective (past) XXXX that XXX already XXXX incurred and cannot XX XXXXXXXXX. XXXX XXXXX XXX XXXXXXXXX XXXXXXXXXX XXXX prospective costs while as marginal XXXX is the XXXXXX in the total cost that arises when the XXXXXXXX XXXXXXXX XXX an XXXXXXXXX by XXXX. XXXX is, it is the cost of producing XXX more unit of a good. XX XXXXXXX terms, marginal XXXX at XXXX XXXXX XX XXXXXXXXXX XXXXXXXX any additional costs XXXXXXXX to XXXXXXX XXX next XXXX.