Which of the following statements is correct concerning the auditor's use of statistical sampling?

a. An assumption of PPS sampling is that the underlying accounting population is normally distributed.

b. An auditor needs to estimate the dollar amount of the standard deviation of the population to use classical variable sampling.

c. A classical variable sample needs to be designed with special considerations to include negative balances in the sample.

d. The selection of zero balances usually does not require special sample design considerations when using PPS sampling.