During 1915 and 1917, u-boats attacked not only enemy ships but also
troops stationed close to shore.
cities near the shoreline.
german submarines.
commercial supply ships.

During 1915 and 1917, U-boats attacked not only enemy ships but also commercial supply ships.

Explanation:

Underwater warfare during the First World War was in part a struggle between the German U-Boats and supply convoys across the Atlantic bound for Great Britain. The British and allied submarines carried out a wide spectrum of operations in the Baltic Sea, the North Sea, the Atlantic Ocean, the Mediterranean Sea and the Black Sea. Only a few actions took place outside the North Atlantic. Attacks by German submarines on Allied merchants gave the United States a direct reason to enter the war in April 1917.

the answer to your question is:

strong-mayor system

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