The assignment of U.S. citizens to Social Security numbers is an example of a function.
Step-by-step explanation:
In order to determinate if the given example is a function, you have to use the definiton of function.
A function assigns each element of the domain (one set) to exactly one element of the range (the other set).
In the given example, the domain is the U.S. citizens and the range is the Social Security numbers. It's a function because every citizen is assigned to exactly one Social Security number.
