(A) the Lusitania
(B) ** U-boats
(C) the Maine
(D) American Expeditionary Force
EXPLANATIONS BELOW
Concept note-1: -On January 31, 1917, Germany announces the renewal of unrestricted submarine warfare in the Atlantic as German torpedo-armed submarines prepare to attack any and all ships, including civilian passenger carriers, said to be sighted in war-zone waters.
Concept note-2: -Germany retaliated by using its submarines to destroy neutral ships that were supplying the Allies. The formidable U-boats (unterseeboots) prowled the Atlantic armed with torpedoes. They were Germany’s only weapon of advantage as Britain effectively blocked German ports to supplies.
Concept note-3: -The term is an anglicised version of the German word U-Boot [ˈuːboːt] ( listen), a shortening of Unterseeboot (under-sea boat), though the German term refers to any submarine. Austro-Hungarian Navy submarines were also known as U-boats.
Concept note-4: -A German U-boat torpedoed the British-owned steamship Lusitania, killing 1, 195 people including 128 Americans, on May 7, 1915.