input: Write a function to create a basic REST API using Python and Flask that lists all the countries in the world. The function should initialize a Flask app and define a route for '/countries' that reads a list of countries from a JSON file named 'data.json' and returns it as a JSON response. The Flask app should run when the main module is executed.
  
  
  assert create_countries_api() == "App running..."
  
output: def create_countries_api():
      from flask import Flask, jsonify
      import json
  
      app = Flask(__name__)
  
      @app.route('/countries')
      def get_countries():
          with open('data.json', 'r') as file:
              countries = json.load(file)
          return jsonify(countries)
  
      if __name__ == '__main__':
          app.run(debug=True)
          return "App running..."