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