Which coat of arms belongs to the Free Hanseatic City of Bremen?

EDITORIAL LEARNING AID

Why is this answer correct?

The Bremen coat of arms shows the silver key on a red background; it refers to St Peter, the patron saint of Bremen Cathedral. Image 3 shows this coat of arms. The other images show coats of arms of other federal states or cities and are therefore wrong.

Bremen: the key of St Peter – Hamburg: the white castle.

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