Ed's Appliances Ltd (Ed's)sold a Furnace to a Customer in October

Ed's Appliances Ltd (Ed's) sold a furnace to a customer in October, 2020 for $10,000. The mark-up on the $5,000 cost of the furnace was 100 percent. The customer paid 20 percent of the purchase price on delivery, with the remainder of the purchase price due in March 2021. What is the maximum reserve Ed's can claim for tax purposes for its November 30, 2020 year end?

A) $2,000.
B) $4,000.
C) $6,000.
D) $8,000.
E) None of the above.