Nine-year-old Brandon has no problem understanding that the 20-year-old woman who sometimes stays with him is both a student and a babysitter. According to Piaget, this is because Brandon's thinking has moved beyond:

The controlled use of fire by hominids: (a) limited the expansion of hominids into certain environments, such as dry grasslands. (b) decreased the amount of food energy available in the hominid diet. (c) contributed to geographical expansion and food production techniques in positive ways. (d) has been shown to contribute little to the digestive process of hominids.