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.

Ex Input:
monosemy is when the words in a language each have only one meaning.

Ex Output:
monosemy


Ex Input:
regent's canal is a canal in england.

Ex Output:
regent's canal


Ex Input:
mariner 10 was a robotic nasa space probe.

Ex Output:

mariner 10