Choice "A" is correct. Inclusion of negative balances requires special design considerations with PPS sampling, but it does not require special design considerations with classical variables sampling.
Choice "c" is incorrect. All items in the population should have an equal chance to be included in the sample. Therefore, the sampling process should not begin before the complete population is available, regardless of whether classical variables sampling or PPS sampling is used.
Choice "d" is incorrect. When planning a particular sample for a substantive test of details, the auditor should consider preliminary estimates of materiality.
Choice "b" is incorrect. If no errors are expected, PPS sampling generally requires a smaller sample than other methods.