Which of the following is used by the text author to explain why people exhibit differences in metabolic efficiency?

A) Metabolic efficiency is advantageous for surviving in an environment where calories are hard to obtain.
B) Metabolic efficiency results in obesity but does not result in health disorder.
C) The excessive accumulation of fat is a social proxy for wealth.
D) Obesity allowed hunter-gatherers to forage for long periods of time.
E) Genetic mutations involving metabolic efficiency are quite common.

Answer: A
Rationale: Metabolic efficiency is advantageous for surviving in an environment where calories are hard to obtain.