(A) ** U-boats
(B) USS Maine
(C) Artillery
EXPLANATIONS BELOW
Concept note-1: -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.
Concept note-2: -Germany has commissioned over 1, 500 U-boats (German: Unterseeboot) into its various navies from 1906 to the present day. The submarines have usually been designated with a U followed by a number, although World War I coastal submarines and coastal minelaying submarines used the UB and UC prefixes, respectively.
Concept note-3: -U-boat, German U-boot, abbreviation of Unterseeboot, (“undersea boat”), a German submarine. The destruction of enemy shipping by German U-boats was a spectacular feature of both World Wars I and II.
Concept note-4: -Despite their prevalence during World War I and World War II, only four U-boats remain intact today.
Concept note-5: -In 1914, Germany had just 20 U-boats. By 1917, it had 140 and the U-boats had destroyed about 30 percent of the world’s merchant ships.