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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def GetClientOs(client_id, token=None):
 """Returns last known operating system name that the client used."""

  if data_store.RelationalDBEnabled():
    kb = data_store.REL_DB.ReadClientSnapshot(client_id).knowledge_base
  else:
    with aff4.FACTORY.Open(client_id, token=token) as client:
      kb = client.Get(client.Schema.KNOWLEDGE_BASE)

  return kb.os