Evil aliens have decided to kill everyone on Earth! Fortunately, they've given you two minutes to save as many as you can. All you have to do is name a country to save everyone in it.
The Ebro itself doesn't flow through Andorra, however it's tributary of the Segre River does. Still, the Ariège/Garonne Rivers are significantly longer, sorry I missed that.
The longest river in that country is Ariège/Arieja