The correct answer to the question is option B
THE COUNTRIES HAD AN EXISTING ALLIANCE.
After Archduke Franz Ferdinand was assassinated, Austria issued Serbia an ultimatum by making a list for the Serbians.
Austria-Hungary actually started the war,this they did by declaring war against Serbia for assassinating Archduke Franz Ferdinand.
Some of the nations they had an alliance with like Germany had no choice than to come along for the ride whereas those that had alliance with Serbia jumped to their defence.
Thereafter,the countries allied with the countries that who were then allies had to come into the war and then everyone in Europe all have an alliance with one side or another excluding the Swiss,this brought about the "The Great War"
Everyone entered this great war because they were bound to eventhough it was something that had nothing to do with their own lives.
