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.

Q: ronald ross (13 may 1857 – 16 september 1932) was an english doctor.

A: ronald ross
****
Q: the adelaide cup is a horserace, held in adelaide, south australia.

A: adelaide cup
****
Q: a hot air balloon is a type of aircraft.

A:
hot air balloon
****