On 29th September 1918, six weeks before the end of the war, the first of the Central Powers left the war after signing the Armistice of Thessalonica. Which one of the Central Powers was it?

The First World War began on 28th July 1914 and lasted for 4 years, 3 months and 2 weeks.  During that time almost 10 million servicemen and nearly 6 million civilians were killed and over 21 million were wounded.  Almost 2% of the world's population died as a direct result of this truly horrific conflict.