Which South American city is famously situated at the mouth of the Rio de la Plata?

Buenos Aires

From which root vegetable were early jack-o'-lanterns traditionally carved before pumpkins became popular? Which artist is famous for the phrase "Turn Down for What"? Which President oversaw the completion of the transcontinental railroad? What controversial policy excluded Chinese immigration to America from 1882? Who was shot while making an assassination attempt on President Gerald Ford?