The state capital of North Rhine-Westphalia is called …

EDITORIAL LEARNING AID

Why is this answer correct?

Düsseldorf is the state capital of North Rhine-Westphalia; the state parliament and the state government have their seat there. Cologne is the largest city in the state, Bonn was the seat of the Bundestag and the federal government until 1999, and Dortmund is a centre of the Ruhr area.

Düsseldorf: state parliament and government, not Cologne.

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