Use reasoning to lead to the answer of the following question:
A battleship can be sunk by an unseen torpedo fired by a what?\nOptions:\n- patrol boat\n- submarine\n- corvette\n- destroyer\n- frigate
 Reasoning process:
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Submarine is an underwater vessel. Submarine cannot be seen from above the water. Submarine can fire torpedoes from underwater. Therefore, the final answer is submarine.
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