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 'nine months'? Context: duration of exactly nine months, regardless of the length in days of a given calendar month, but typically 30 days

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

Answer: proper interval