Question

in december 1917, russia dropped out of world war i. what was the main event that caused this?
the influenza epidemic
the industrial revolution
the invention of the atomic bomb
the russian revolution

Explanation:

The influenza epidemic occurred later (1918-1919), the Industrial Revolution was in the 18th-19th centuries, and the atomic bomb was invented in the 1940s. The 1917 Russian Revolution led to the Bolshevik government, which sought to end Russia's involvement in WWI and signed the Treaty of Brest-Litovsk in March 1918 (following the December 1917 decision to withdraw).

Answer:

the Russian Revolution