The graph below illustrates the results of an investigation using newborn rats. The dashed line curve shows the normal growth rate of rats, based on previous investigations. The solid-line curve shows the growth rate of 50 rats that were fed a normal diet containing nutrients A and B until they reached an age of 80 days (point X). At point X, the amounts of nutrients A and B were increased. At point Y, nutrient A was returned to the normal amount. At point Z, nutrient B was returned to the normal amount. According to the graph, at what age do rats normally reach a stable weight?