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.

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

the palaeoptera is the group of primitive (basal) winged insects which do not fold the wings back over the abdomen, as neoptera do.
palaeoptera

a candela is the si base unit of luminous intensity.
candela