input: from typing import List
  
  def list_countries(countries: List[str]) -> str:
      """ Given a list of country names, return a string of these names concatenated by commas.
      Example:
      >>> list_countries(['USA', 'Canada', 'Mexico'])
      'USA, Canada, Mexico'
      >>> list_countries(['China', 'Japan', 'India'])
      'China, Japan, India'
      """
output:     return ', '.join(countries)