History: What are the primary factors we use to determine the distance of a sound?

What are the primary factors we use to determine the distance of a sound?

• Comparative amplitude between ears, sound arriving faster at one ear.
  This is used normally for the direction of the sound.
• Volume, high-frequency content.
  Correct.
• None of the other answers.
  There is a good answer available.
• Change in pitch.
  We use this for the movement of the sound.