J. Smith was preparing the December 31 year-end financial statements for her company, Smith’s Used Vehicles. On the December 25, the company sold a car that it had not yet paid for. J. Smith did not record the cost of the car as an expense in the year-end income statement because she did not think it was appropriate to expense something that had not yet been paid for. Which of the following fundamental concepts of accounting has she failed to follow? ( )