Which most accurately explains how the union of Austria and Hungary contributed to the outbreak of World War I?

The war was a global conflict. Thirty-two nations were eventually involved. Twenty-eight of these constituted the Allied and Associated Powers, whose principal belligerents were the British Empire, France, Italy, Russia, Serbia, and the United States of America. They were opposed by the Central Powers: Austria-Hungary, Bulgaria, Germany, and the Ottoman Empire.

Expansion of the war was swift. Austria-Hungary declared war on Serbia on 28 July 1914;