Answer:
A Â - Alliances caused more and more countries to join the war
Explanation:
https://alphahistory.com/worldwar1/alliances/
To sum it up, Germany and Austria-Hungary where allies. There alliance was called the Central Powers. Serbia was a friend of Russia, so when Austria-Hungary invaded, Russia joined the war against the Central Powers. Russia was allied to France. Germany wanted to take out France fast and then focus on Russia. They went around the French border and invade Belgium. Belgium independence was guaranteed by the United Kingdom thus pulling them in the war.