What should be the ideal distance between your head and the head restraint if it tilts?

• More than 20 centimetres
• More than 25 centimetres
• Less than 10 centimetres
  Explanation:

  "If your head restraint tilts, adjust it forward to reduce the gap between your head and the restraint. The ideal distance is less than 10 centimetres (4 inches) for maximum protection against whiplash injuries."
• Less than 50 centimetres