input: Write a function to create a basic REST API using Python and Flask to list all the countries in the world. If the API is successfully created, return the Flask app instance, otherwise return None.
  
  import flask
  from flask import Flask, request, jsonify
  import json
  
  def create_country_api():
      app = Flask(__name__)
  
      @app.route('/countries', methods=['GET'])
      def get_countries():
          with open('data.json', 'r') as f:
              countries = json.load(f)
          return jsonify(countries)
  
      if __name__ == '__main__':
          app.run()
      
      return app if app is not None else None
  
  assert create_country_api() is not None
  assert callable(create_country_api())
  assert create_country_api().name == '__main__'
output: def create_country_api():
      app = Flask(__name__)
  
      @app.route('/countries', methods=['GET'])
      def get_countries():
          with open('data.json', 'r') as f:
              countries = json.load(f)
          return jsonify(countries)
  
      return app if app is not None else None