When did the GDR build the Wall in Berlin?

EDITORIAL LEARNING AID

Why is this answer correct?

The GDR built the Wall on 13 August 1961 in order to stop people fleeing to the West. 1919 and 1933 lie before the founding of the GDR in 1949. By 1990 the Wall had long been opened and the GDR acceded to the Federal Republic.

In 1961 the GDR built the Wall.

This translation and explanation are editorial learning aids. The official German wording and answer key govern the examination.