A. Alliances meant that if one country was attacked, many more would be drawn into war.
One of the main causes of the beginning of the war was the alliance between the European nations, which meant that they would defend each other.
Following the assassination of Archduke Franz Ferdinand (the heir presumptive of the Austro-Hungarian throne) by a Serbian terrorist group, Austria-Hungary declared war on Serbia on July 1914. Russians, as an allied to Serbia, got involved in the war too.
The Russian involvement caused that Germany (allied to Austria-Hungary) declared war on Russia. France, allied to Russia, intervened to defend it. Then Germany's decision to attack France through Belgium drew Britain into war, which also led Japanese to get involved since they were allies with Britain.
And even though Italy and the United States had declared themselves neutral at the beginning, they both ended up entering as well. Italy joined in April 1915 and the United States in April 1917.