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C is corrent. The requirement is to identify when weighted-average and first-in, first-out FIFO equivalent units would be the same in the period. C is corrent because the only difference between the two methods is that FIFO segregates the beginning inventory costs and assumes that they move out first. If there is no beginning inventory, weighted-average and FIFO costs are the same.
A is incorrect because if a beginning inventory exists at all, it is not likely that the costs would be the same.
B is incorrect because it is the beginning inventory costs that make the difference, not the number of units.
D is incorrect because whether or not there is an ending inventory is irrelevant.