Correct answer is C. Nations had to join the war to help their allies.
Explanation:
Option C is correct because after the war started countries started supporting each other. Germany was supported by Germany, Russia by France and Britain, and in time half of Europe was involved in war.
A is not correct as more countries than ever participated in war.
B is not correct as this war lasted more than four years.
D is not correct as countries had to focus on their own resources a lot, not only on colonies.
