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:
the features of a production vehicle or production car are mass-produced identical models, offered for sale to the public.

Ex Output:
production vehicle


Ex Input:
san diego creek is a small northwest-flowing river in orange county, california.

Ex Output:
san diego creek


Ex Input:
the macbook air is a kind of macintosh laptop computer designed by apple, known as the "world's thinnest notebook."

Ex Output:

macbook air