A Multiple Choice Quesion

The following is a question from FA2 Revision Kit: Which one of the following source documents is summarized and posted to the general ledger? A. Sales returns day book B. Purchases day book C. Cash recieved day book D. The purchase returns day book The kit says C is right. The other three are posted to the receivables /payables control accounts. ---------------------------------------------------------------- My Thoughts 1. I think none of the above four options is a source document. All of them are books of prime entry. 2. I agree that the other three are posted to the receivables /payables control accounts, but these accounts are also kept in general ledger. I think all are posted to general ledger, not just C. I think the whole question is wrong. Need your help! Thanks! ?