Given the task definition and input, reply with output. Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.

Context: The Lambro (Western Lombard: Lamber or Lambar) is a river of Lombardy, northern Italy, a left tributary of the Po.
Question: What river is Lambro a tributary of?
po