Answer:
a) FIFO
Step-by-step explanation:
FIFO means first in, first out. It is an inventory system where the first purchased inventory is the first to be sold . The cost of goods sold is $30 which is equal to the price of the first purchased inventory . Therefore, the FIFO inventory system was used.
LIFO means last in, first out. It is an inventory system where the last purchased inventory is the first to be sold.
Weighted average is when the weighted price of inventory is used as the cost of goods sold.
I hope my answer helps you.