input: Write a python function to print 'Hello World' if the input is 'Y' and 'Goodbye' if the input is 'N'.
  assert user_response('Y') == 'Hello World'
  assert user_response('N') == 'Goodbye'
  assert user_response('A') == None
output: def user_response(input):
      if input == 'Y':
          return 'Hello World'
      elif input == 'N':
          return 'Goodbye'
      else:
          return None