A Reason(s)why "Pure" FIFO Is Rarely Encountered in Process Costing

A reason(s) why "pure" FIFO is rarely encountered in process costing is that:

A) FIFO is usually applied within a department to compute the cost of units transferred out
B) the units transferred into the department during a given time period are usually carried at a single average unit cost
C) tracking costs on a "pure" FIFO basis is very difficult
D) All of these answers are correct.