Some of the objectives of the accounts payable function may be regarded as making sure that

(a) goods and services to be paid for have been received,

(b) amounts billed are contractually authorized,

(c) amounts due are mathematically correct, and (d)

amounts approved for payment are distributed to the appropriate accounts.

For each of these four objectives, list two key steps you would include in your audit program to determine whether the controls designed to achieve those objectives are operating effectively.