External Auditing question - substantive procedures or test of controls or both

I'm hoping someone is able to answer this.
The question comes from the osborne workbook practice assessment 2 and I don't understand the reasoning behind the solutions in the the back.

identify the most likely approach to be taken by an auditor in the following circumstances;
a) audit work in a company where records are fully computerised and maintained by the accountant and an assistant
b) verifying the valuation of trade payables
c) reviewing a provision for doubtful debts
d) carrying out the audit of a company which has an internal audit function which monitors operational and financial controls

Your choices for each are out of the below;
1 - test of controls only
2 - substantive procedures only
3 - a mixture of test of controls and substantive procedures

Please could you give your reasonings behind your choices.
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