Which of the following statement about long-term costs is true?( )

A、Long-term costs are divided into fixed costs and variable costs
B、When the long-term average cost is equal to the marginal cost, the long-term average total cost is the highest
C、The long-term average cost function is suitable for situations where all inputs are fixed costs in the long run
D、There are no fixed costs in long-term costs; all costs are variable costs.