Answer:
B. Alliances
Step-by-step explanation:
Alliances between major countries made sure they both defended each other's land and provide aid whether its ammunition or medical supplies. The two major alliances during WW1 was the Allies and Central Powers.
Triple Entente:
-British Empire
-France
-Russian Empire
Other Allies with the Triple Entente:
-Kingdom of Italy (Used to be allies with Germany in the Triple Alliance)
United States (Joined in 1917)
-Kingdom of Serbia
Central Powers:
German Empire
Austria-Hungary Empire
Ottoman Empire
Bulgaria