Which one of the following is TRUE with respect to the estimate of vacancy rates in the appraisal of income-producing properties?
a-Vacancy rates are expressed as a percentage of net operating income.
b- Vacancy rates are considered an operating expense.
c- Vacancy rates can be ignored if the subject property has never had any vacancies.
d-Vacancy rates are needed because the 100% occupancy assumption of gross potential revenue is unrealistic for most properties.