Generate a topic word for the given sentence. A topic word is usually the name of a person, place, animal, concept or object the sentence is focusing on.

Example Input: a releaser is a stimulus from one animal to another, which causes a particular response.
Example Output: releaser

Example Input: a rearguard is a part of a military force that protects the main force from attack from the rear.
Example Output: rearguard

Example Input: hurricane audrey was the most powerful june hurricane in the recorded history of the atlantic since record keeping began.
Example Output:
hurricane audrey