(A) ** German u-boat attacks
(B) Austrian u-boat attacks
(C) Russian u-boat attacks
(D) English u-boat attacks
EXPLANATIONS BELOW
Concept note-1: -The entry of the United States was the turning point of the war, because it made the eventual defeat of Germany possible. It had been foreseen in 1916 that if the United States went to war, the Allies’ military effort against Germany would be upheld by U.S. supplies and by enormous extensions of credit.
Concept note-2: -Britain’s blockade across the North Sea and the English Channel cut the flow of war supplies, food, and fuel to Germany during World War I. Germany retaliated by using its submarines to destroy neutral ships that were supplying the Allies.
Concept note-3: -U-boats were naval submarines operated by Germany, particularly in the First and Second World Wars. Although at times they were efficient fleet weapons against enemy naval warships, they were most effectively used in an economic-warfare role (commerce raiding) and enforcing a naval blockade against enemy shipping.