Why did german u-boats sink unarmed allied ships such as the lusitania?

On February 4, 1915, Germany declared the maritime zone around the British Isles as a war zone: as of February 18, allied ships present in that area could be sunk without warning. This position was not, however, an indiscriminate underwater war in the first place, since efforts were made to avoid sinking neutral ships.

On April 23, the German embassy in the USA published a notice in the newspapers strangely coinciding with the departure of the Lusitania. In this, it warned passengers about the risk of sailing in non-neutral waters due to the risk of being attacked. Only one newspaper managed to answer the German notice in its editions. Precisely the route of the Lusitania passed through hostile waters. This circumstance was not unknown either to the Cunard shipping company, or to the English Admiralty, and even less to the captain of the ship William Thomas Turner.

On May 1, 1915 at 11:30 the RMS Lusitania left Pier No. 54 in the port of New York and headed for the Atlantic with 1,959 passengers on board, including 136 American passengers, 129 children and 39 babies.

On May 7, the Lusitania was identified and torpedoed by a U-boat and sank in 18 minutes, near the Cape of Old Head of Kinsale, killing 1,198 people and leaving 761 survivors.

they supported black codes so the answer is a or b