def _GetExtentField(json_info):
  """The extent field must be only one of extent or wgs84Extent."""
  has_extent = 'extent' in json_info
  has_wgs84 = 'wgs84Extent' in json_info

  if has_extent and not has_wgs84:
    return 'extent'
  if not has_extent and has_wgs84:
    return 'wgs84Extent'

  return None