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 breed is a domesticated race or type of an animal or plant.
Example Output: breed

Example Input: malva is a genus of about 25-30 species of plants commonly called mallows.
Example Output: malva

Example Input: the household income in the united states is a measure of current private income commonly used by the united states government and private institutions.
Example Output:
household income in the united states