Task: Inferred Instance Class Reasoning (ontology: Business_Finance/Financial_Industry_Business_Ontology)

After reasoning over the ontology, which class is one valid inferred class for the instance 'one hundred eighty months'? Context: duration of exactly one hundred eighty months, regardless of the length in days of a given calendar month, but typically 30 days

A. proper interval
B. recurrence interval
C. calendar period
D. date period

Answer: proper interval