Who built the Wall in Berlin?

EDITORIAL LEARNING AID

Why is this answer correct?

The Berlin Wall was built by the GDR on 13 August 1961 in order to stop its citizens from fleeing to the West. Great Britain and the USA were protecting powers of West Berlin and did not build a wall. The Federal Republic of Germany was the western part of the country and did not seal off its borders in this way.

In 1961 the GDR built the Wall, not the Western Allies.

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