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.
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Question: zirconium is a chemical elemental metal.

Answer: zirconium


Question: hair straightening is when a person makes their hair straighter.

Answer: hair straightening


Question: hildesheim is one of the oldest cities in north germany.

Answer:
hildesheim