Can anyone explain to me why my choice is incorrect?

Question 15 (INVY-0045) The calculation of the income recognized in the third year of a five-year construction contract accounted for using the percentage-of-completion method includes the ratio of Costs incurred in year 3 to total billings. Your Answer Costs incurred in year 3 to total estimated costs. Total costs incurred to date to total billings. Correct Total costs incurred to date to total estimated costs. This Answer is Correct This answer is correct. The amount is computed as the ratio of total costs incurred to date to the total estimated costs