A battleship can be sunk by an unseen torpedo fired by a what?
Options:
- patrol boat
- submarine
- corvette
- destroyer
- frigate
Let's think carefully first. Step-by-step reasoning process:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
Submarine is an underwater vessel. Submarine cannot be seen from above the water. Submarine can fire torpedoes from underwater. So the final answer is submarine.