def _tpu_zone(index):
  """Chooses a GCP TPU zone based on the index."""
  if index < 70:
    return 'europe-west4-a'
  elif index < 80:  # 10
    return 'us-central1-b'
  elif index < 90:  # 10
    return 'us-central1-c'
  elif index < 100:  # 30 (seems like 10...)
    return 'asia-east1-c'
  elif index < 110:
    return 'us-central1-a'
  elif index < 120:
    return 'us-central1-b'
  elif index < 130:
    return 'europe-west4-a'
  else:
    raise ValueError('Unhandled zone index')