audit procedure

Materiality has been set at $50000. Audit manager has emphasised the understatement of trade payables balance is a significant audit risk.
elow is extract from list of supplier statement as at 30 june 20X6 held by company and corresponding payables ledger balances at the same date along with some commentary on the noted differences

Supplier Statement balance payable ledger bal
Carnation co 70000 50000
Lily co 175000 105000

Carnation co
The difference in the balance is due to an invoice which is under dispute dur to faulty goods which were returned on 29 june 20X6

Lily co
The difference in the balance is due to supplier statement showing an invoice dated 28 june 20×6 for$70000 which was not recorded in financial statement until after year end. The payable clerk has advised the audit team that the invoice was not recieved until 2 july 20×6

Audit manager has asked you to review full list of trade payables and select balances on which supplier statement
reconcilations will be performed

Which of the following audit procedures should be carried out to confirm balance owing to Carnation co
a)Review post year end credit notes for evidence of acceptance of return
b)Inspect pre year end goods returned note in respect of items sent back to the supplier
c)Inspect post year end cash book for evidence that the amount has been settled

Sir correct ans is a and b. But please can you explain