Which of the following would provide the best evidence of compliance with an airline’s standard of having aircraft refueled and cleaned within a specified time of arrival at an airport?

A. Vendor fuel invoices that have been reconciled to inventory records.
B. Time cards completed by aircraft cleaning and fueling crews.

C. Observation of selected aircraft while they are being refueled and cleaned.

D. Comparison of the standard hourly labor costs for cleaning and fueling personnel with actual labor charges.