Why did the battle of tannenberg occur?

World War I1914Battle of Tannenberg beginsShare this:facebooktwittergoogle+PRINT CITEOn August 26, 1914, the German 8th Army, under the leadership of Paul von Hindenburg and Erich Ludendorff, strikes with lethal force against the advancing Russian 2nd Army, led by General Aleksandr Samsonov, in East Prussia during the opening weeks of the First World War.In the middle of August 1914, much sooner than had been anticipated, Russia sent two armies into East Prussia, while Germany, according to its war strategy, had the bulk of its forces concentrated to the west, against France. The Russian 1st Army, under General Pavel Rennenkampf, advanced to the northeastern corner of East Prussia, while Samsonov’s 2nd Army made headway into the southwest, planning to join with Rennenkampf’s men and pin the outnumbered German 8th Army between them. After a Russian victory in the Battle of Gumbinnen on August 20, however, Rennenkampf paused to regroup his forces.

The answer is c because i know the answer is c