Which of the following is an example of arbitrage?
A.A firm sells a box of cereal at $10 when the average cost of producing it is $6.
B.Thomas buys a new stock issued by a firm on the stock exchange
C.A local salon charges 5 percent more for all its services than a competing salon in the same locality.
D.Romi buys a DVD from Walmart at $10 and sells it on eBay for $20.