Which of the following sets of duties would ordinarily be considered basically incompatible in terms of good internal control?

a. Custody of unmailed signed checks and maintenance of expense subsidiary ledgers.

b. Posting to the general ledger and approval of additions and terminations relating to the payroll.

c. Preparation of monthly statements to customers and maintenance of the accounts receivable subsidiary ledger.

d. Collection of receipts on account and maintaining accounts receivable records.