keeping in mind that a function does not have any X-Repeats, that is, the 1st coordinate in every pair never repeats, well, in this case, all of them have no X-repeats, that means all of them are functions. However, only one-to-one functions have an inverse that is also a function.
A one-to-one function is one that has no X-repeats as well as no Y-repeats, so each pair in the set has no coordinate repeats, Check the picture below.
