The following costs are related to plane trips between Portland, Oregon and Charlotte, North Carolina. Pilots are paid on a per-trip basis. Identify the costs as fixed or variable.

• Pilots salaries relative to the number of trips flown.
• Pilots salaries relative to the number of passengers on a particular trip.
• Cost of refreshments relative to the number of passengers.
• Cost of a maintenance check relative to the number of passengers on a particular trip.
• Fuel costs relative to the number of trips.