(A) isolationism
(B) rationing
(C) atomic bombs
(D) ** submarine warfare
EXPLANATIONS BELOW
Concept note-1: -The German use of U-boat caused tension to increase between the United States and Germany. Explanation: The Germans used their most important naval weapon which was the U-boat.
Concept note-2: -The Sussex Pledge was a promise made by Germany to the United States in 1916, during World War I before the latter entered World War I. Early in 1915, Germany had instituted a policy of unrestricted submarine warfare, allowing armed merchant ships but not passenger ships to be torpedoed without warning.
Concept note-3: -Answer and Explanation: The main result of Germany’s use of unrestricted submarine warfare during World War l was the entry of the United States into the conflict. Submarines had been attacking the ships of neutral countries, as well as, ships that carried civilian passengers.
Concept note-4: -On May 4, 1916, Germany responds to a demand by U.S. President Woodrow Wilson by agreeing to limit its submarine warfare in order to avert a diplomatic break with the United States.
Concept note-5: -The United States later declared war on Austria-Hungary on December 7, 1917. Germany’s resumption of submarine attacks on passenger and merchant ships in 1917 was the primary motivation behind Wilson’s decision to lead the United States into World War I.