Question

Which factor most contributed to the United States' decision to enter World War 1?
A. Russia's exit from the war after its government collapsed
B. The Ottoman Empire launching the Armenian genocide
C. The Allied powers' failure in the Gallipoli Campaign
D. German submarine attacks on merchant ships

Answer 1

the answer is simply D)
German submarine attacks on merchant ships
there you go

Answer 2

D. German submarine attacks on merchant ships

or

Germany attacking merchant ships that carried American civilians