Since when have people been paying with the euro in cash in Germany?

EDITORIAL LEARNING AID

Why is this answer correct?

Euro cash has existed in Germany since 1 January 2002; before that, people paid with Deutschmarks. The euro was already introduced as book money in 1999, and by 2005 the cash had long been in circulation.

Euro cash: since 1 January 2002.

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