Could you please tell me how to calculate and why? Thanks a lot.

At December 1, 2014, Gibson Company's accounts receivable balance was 2,400. During December, Gibson had credit revenues of 10,000 and collected accounts receivable of 8,000. At December 31, 2014, the accounts receivable balance is a. 2,400 debit. b. 4,400 debit. c. 12,400 debit. d. 4,400 credit