GAAP and IFRS allow various treatments for revenue recognition. Which of the following correctly describes acceptable revenue recognition principles?

a. Whenever cash is realized in a transaction with a customer (such as when a firm sells an asset to a customer)
b. When value changes occur over time but are realized in a market transaction
c. When the firm has completed all or substantially all of the revenue-generating processes by delivering goods or services to customers, and it is reasonable that it has generated an asset or satisfied a liability
d. All of these answer choices are correct, depending on the circumstance