What effects did submarines (U-Boats) in ww1

Germany retaliated by sinking neutral ships supplying the Allies with its submarines. The fearsome U-boats (unterseeboots), armed with torpedoes, patrolled the Atlantic. 

As Britain effectively barred German ports from imports, they were Germany's only weapon of advantage.

answer; ///i believe that the correct answer is (a); ///the crystal will

the answer is c my brotha. c.   formed an alliance with the soviet union.