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.
--------
Question: a viola is a musical instrument which is similar to the violin and cello.

Answer: viola


Question: a marriage proposal is when one person in a relationship asks the other person to marry them.

Answer: marriage proposal


Question: stornoway airport is an airfield on the outer hebrides isle of lewis.

Answer:
stornoway airport