def _zone(index):
  """Chooses a GCP zone based on the index."""
  if index < 6:
    return 'us-central1-a'
  elif index < 12:
    return 'us-east1-b'
  elif index < 18:
    return 'us-east4-c'
  elif index < 24:
    return 'us-west2-a'
  else:
    raise ValueError('Unhandled zone index')