Final answer:
The Ottoman Empire and Bulgaria were the two countries that joined Austria-Hungary and Germany to form the Central Powers during World War I.
Step-by-step explanation:
The two countries that joined Austria-Hungary and Germany to form the Central Powers during World War I were the Ottoman Empire and Bulgaria. The Ottoman Empire entered the war on the side of the Central Powers by attacking Russia's fleet on the Black Sea in October 1914.
Bulgaria joined the war by a secret agreement in 1915, also aligning with the Central Powers. These additions sought territorial gains and were strategically significant in the larger conflict that pitted them against the Allied Powers, which included Great Britain, France, and Russia.