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: bothnian bay (; ) is a bay in the northest of the gulf of bothnia.

A: bothnian bay
****
Q: the british rail class 931 was reserved for former southern region electrical multiple units converted for departmental use.

A: british rail class 931
****
Q: hair straightening is when a person makes their hair straighter.

A:
hair straightening
****