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.

Input: Consider Input: the alkaline earth metals are the second group of metals on the periodic table.

Output: alkaline earth metal

Input: Consider Input: the 1st millennium bc was the last millennium before the common era.

Output: 1st millennium bc

Input: Consider Input: the title of duke of nemours was originally given to a french family of minor nobility.
Output: duke of nemours