Concept:The oestrus cycle is the time from one heat period to the next heat period in the female animal.Explanation:The sow has an average oestrus cycle of about 21 days. It usually falls within a range of 18 to 24 days. Among the given options, 14−28 days best covers this typical range. Other options are too short for a sow's cycle.Answer:A. 14−28 days