What are external cues/signals and how does it affect hunger?

Based on the graph, how is coal consumption likely to affect future generations? a. the higher our current rate of consumption, the less coal will be available in the future. b. current rates of consumption are irrelevant because new coal will form in the future. c. the lower our current rate of consumption, the more slowly coal will be replenished for the future. d. regardless of current rates of consumption, coal will always be available.