Does the tagus river has its source or mouth in Spain?

The correct answer is source.

The Tagus River (Portuguese: Tejo [ˈtɛʒu]; Spanish: Tajo [ˈtaxo]) is the longest river of the Iberian Peninsula. Rising in the Montes Universales in east-central Spain, it flows westwards through central Spain, then southwards, along the border between Spain and Portugal, before emptying into the Atlantic Ocean at Lisbon, Portugal.

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