If the function h(x) represents the number of full hours that it takes a person to assemble x sets of tires in a factory, which would be an appropriate domain for the function?
1) the set of real numbers
2) the set of negative integers
3) the set of integers
4) the set of non-negative integers
why?

Of course the D answer is correct.
The number of sets of tires must be a whole number. It can't be real number because it is impossible to assemble for example 3.2321 set of tires.The negative -- it certainly also isn't possible. The amount can't be a negative number that's why C isn't correct as well.