Which of the following statements is true according to these road signs?

• The speed limit is 25 miles per hour only in residential areas.
• The speed limit is 25 miles per hour in citywide areas only.
• The speed limit is 25 miles per hour only in townwide areas.
• The speed limit is 25 miles per hour in both citywide and townwide areas, unless otherwise posted.
  Explanation:

  "The speed limit is 25 miles per hour in both citywide and townwide areas, unless otherwise posted."