Sigma Inc. sold merchandise (Sigmas cost, $2,000), to Theta Inc. for $5,000, terms 1/10, n/30. Five days later Theta returned $1,300 of the merchandise to Sigma (Sigmas cost $520). Required: Prepare the journal entries required to record the following transactions: a) The sale on Sigmas books assuming Sigma uses the periodic inventory system. b) The purchase on Thetas books, assuming Theta uses the periodic inventory system. c) The return on Sigmas books, assuming Sigma uses the perpetual inventory system. d) The payment, with-in the discount period, on Thetas books, assuming Theta uses the periodic inventory system