Question

Let P be a set of people, and let Q be a set of occupations. Define a function
f : P → Q
by setting f(p) equal to p's occupation. What must be true about the people in P for f to be a well-defined function?

1) Everybody must have more than one occupation.
2) Nobody will have only one occupation.
3) Everybody cannot have more than one occupation, but somebody will have more than one occupation.
4) Somebody will have no occupation.
5) Everybody must have some occupation, and nobody can have two occupations.