Question

Which was a cause of World War I?

The Second Industrial Revolution led to international rivalries.

Political revolutions in Europe left nations unstable.

Russia’s race to industrialize threatened the economy of other European countries.


Europe’s system of military alliances enlarged small conflicts.

Answer 1

I believe it's the last one: Europe's system of military alliances enlarged small conflicts. There were a lot of alliances between different European countries at the time, and if you messed with one country, chances are that you got that country's ally involved too...which makes for wars pretty fast. 

Answer 2

Europe's system of military alliances enlarged small conflicts