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A is corrent because the ratio estimation sampling technique is based upon comparing the ratio of the book value to the audited value of the sampled items. This method cannot be used when book values for the sample items are unknown.
B is incorrect because there must be some observed differences between the audit values and the book values to obtain valid conclusions when the ratio estimation sampling technique is used.
C is incorrect because the total book value must be known and it must correspond to the sum of the individual book values if the ratio estimation sampling technique is to be used.
D is incorrect because ratio estimation will be more efficient in terms of required sample size than alternative methods when the audited values are nearly proportional to the book values.