History: Which of the following is why we do not perceive frequencies evenly?

Which of the following is why we do not perceive frequencies evenly?

• We perceive frequencies evenly.
  The unit has the opposite argument to make.
• Complexity and processing ability.
  This is not provided in the text.
• Anatomy and survival.
  Correct!
• Cochlea and hair cells.
  These two parts are involved with hearing, but are part of the how, not the why.