X. X XXXX cost is a XXXXXXXXXXXXX (past) cost that XXX XXXXXXX XXXX XXXXXXXX XXX XXXXXX XX recovered. Sunk costs are sometimes contrasted with prospective costs while as marginal cost is XXX change in the total XXXX that arises when the XXXXXXXX XXXXXXXX XXX an increment XX XXXX. That is, it is the cost XX producing XXX XXXX unit XX a good. XX XXXXXXX terms, XXXXXXXX cost at XXXX level of XXXXXXXXXX XXXXXXXX any additional costs required to produce XXX XXXX unit.