same method to account for COGS and Inventory guess, only, accounting, different, companies

domani

have been getting some questions confused when they ask for differences in LIFO, FIFO inventory, COGS, and these should not be confusing.
to confirm, companies can only use one method of accounting for COGS and inventory, correct?
guess sometimes get mixed up when they say FIFO COGS vs LIFO inventory and think they are using different methods to account for each.
thanks, john