Derek, a limited liability company, operates a computerised purchase system. Invoices and credit notes are posted to the purchases ledger by the purchases ledger department. The computer subsequently raises a cheque when the invoice has to be paid. Required List the controls that should be in operation: (a) Over the addition, amendment and deletion of suppliers, ensuring that the standing data only includes suppliers from the company's list of authorised suppliers (b) Over purchase invoices and credit notes, to ensure only authorised purchase invoices and creditnotes are posted to the purchase ledger