input: 
  def get_countries_as_json() -> None:
      """ Create a basic REST API using Python and Flask to list all the countries in the world.
      The countries data is stored in a JSON file named 'data.json'. The API will respond to GET requests at the '/countries' endpoint.
      
      This function sets up the Flask application, defines the route to list all countries, and runs the application.
      
      Example:
      When the server is running and a GET request is made to '/countries', it should return the list of countries in JSON format.
      """
  
output:     
      from flask import Flask, jsonify
      import json
  
      app = Flask(__name__)
  
      @app.route('/countries', methods=['GET'])
      def get_countries():
          with open('data.json', 'r') as file:
              countries = json.load(file)
          return jsonify(countries)
  
      if __name__ == '__main__':
          app.run(debug=True)
  