input: Write a function to create a basic REST API using Python and Flask to list all the countries in the world. The API should have a route `/countries` that responds with a JSON list of countries when accessed via a GET request. The data should be read from a JSON file named `data.json`.
  
  from flask import Flask, jsonify
  import json
  
  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()
  
  assert get_countries() == jsonify(['Country1', 'Country2', 'Country3'])
  assert get_countries() == jsonify([])
output: 
  def get_countries():
      with open('data.json', 'r') as f:
          countries = json.load(f)
      return jsonify(countries)
  