What does it mean for a set to be "closed"?

Question

A) An operation performed on members of that set will result in a member of the same set.
B) The set has a finite number of members, as opposed to being infinitely large.
C) Members of a closed set cannot be added, subtracted, multiplied, or divided.
D) An expression belonging to a closed set cannot be used to create an expression that is not of that set.

Answer 1

The correct answer is:

A) An operation performed on members of that set will result in a member of the same set.

In mathematical terms, a set is considered "closed" under a certain operation if performing that operation on any members of the set results in an outcome that is also a member of the same set. For example, the set of integers is closed under addition because the sum of any two integers is always an integer.