The combination of imperialism, nationalism, militarism and alliances made Europe ready to explode with one little spark--the assassination of Franz Ferdinand and his wife Sophie on June 28, 1914. Ferdinand was the archduke, or leader, of Austria-Hungary. While visiting Sarajevo, he and his wife were killed by an assassin from Serbia, a country directly east of Austria-Hungary. Because of the assassination, Austria-Hungary declared war on Serbia. But it isn't fair to blame the entire war on the assassination of one fairly unimportant leader. Because Serbia was part of the Allied Powers, Russia stepped in to support Serbia against Austria-Hungary. And Germany wanted to help Austria-Hungary, so it declared war on Russia and France. Then Germany invaded Belgium. England and Japan couldn't stand by while Germany attacked their Russian and French allies, so they declared war on Germany. Driven by nationalism and alliances, and fueled by newly built-up weapons supplies, everyone was ready and able to fight. By the end of the year, the entire continent was at war. According to the text, why did Russia declare war on Austria-Hungary?
A to support Belgium after it was invaded by Germany
B because England and Japan stood by while Germany attacked their allies
C to retaliate after of the assassination of Franz Ferdinand
D to support Serbia, which was also part of the Allied Powers