Which of the following is an inherent difficulty in determining the results of operations on an interim basis?

a. Cost of sales reflects only the amount of product expense allocable to revenue recognized as of the interim date.

b. Depreciation on an interim basis is a partial estimate of the actual annual amount.

c. Costs expensed in one interim period may benefit other periods.

d. Revenue from long-term construction contracts accounted for by the percentage-of-completion method is based on annual completion, and interim estimates may be incorrect.