Answer:
The marginal cost of additional passengers is very small.
Step-by-step explanation:
Marginal cost of adding extra passenger is very low or negligible. Thus, it will be profitable to offer seat to standby customers at discount. If Airline does not offer this seat to standby customers, it will get nothing. Thus, it is rational decision to offer it at discounted rate. Firm has to incur very small extra cost in form of facilities to new customer.