Why does 1/1, 2/2, 3/3 all equal 1 or 1 whole

Imagine you have a pizza, and you don’t cut it. You have one whole pizza, or 1/1 slices. Now imagine that you cut the pizza in half. If you took only one slice you would have half of the pizza, or 1/2. But if you took both slices, you would have the whole pizza, 2/2 slices. A value divided by itself always equals one, no matter what it is.

21 is the answer the answer is 21

What's the denominator