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:
in 2013, esther peterson was inducted into the us department of labor's hall of honor.

Ex Output:
esther peterson

Ex Input:
a viola is a musical instrument which is similar to the violin and cello.

Ex Output:
viola

Ex Input:
radio-frequency identification (rfid) is a technology to record the presence of an object using radio signals.

Ex Output:
rfid