In this task you will be given an answer to a question. You need to generate a question. The answer given should be a correct answer for the generated question.

Example input: This list contains the top 25 accounts with the most followers on the social photo-sharing platform Instagram. As of May 2018, the most followed user is Instagram's own account, with over 235 million followers. Selena Gomez is the most followed individual, with over 137 million followers. Ten accounts have exceeded 100 million followers on the site.
Example output: who has the maximum number of followers on instagram
Example explanation: The answer is talking about the Instagram accounts that have the most followers. The question asking about the maximum number of followers. So this is a good example.
Q: The Battle of Passchendaele (German: Flandernschlacht, French: Deuxième Bataille des Flandres), also known as the Third Battle of Ypres, was a campaign of the First World War, fought by the Allies against the German Empire. The battle took place on the Western Front, from July to November 1917, for control of the ridges south and east of the Belgian city of Ypres in West Flanders, as part of a strategy decided by the Allies at conferences in November 1916 and May 1917. Passchendaele lay on the last ridge east of Ypres, 5 mi (8.0 km) from a railway junction at Roulers, which was vital to the supply system of the German 4th Army. The next stage of the Allied plan was an advance to Thourout -- Couckelaere, to close the German-controlled railway running through Roulers and Thourout.
A:
what was the purpose of the third battle of ypres