When you head to bed at night, you bring a glass of tap water with you and set it on the nightstand. Upon waking in the morning, you discover there are tiny gas bubbles along the sides of the container. The best explanation for this is

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.