Which of the following are possible reason for a credit balance on the sales ledger...

Which of the following are possible reason for a credit balance on the sales ledger account of a customer?

(a) A contra entry between the sales ledger and the purchase ledger has been carried out for an amount in excess of the sales ledger balance

(b) A customer has returned goods subsequent to making payment for them

(c) A credit note has been issued in error

(d) A bad debt written off has subsequently been paid