The definition of a function is that every input, x, must have only one output, y. So, for every x value, there should be only one y value with it. If you have two x values that are the same and have different y values, then that is not a function. The only answer that showcases this is letter D.
<em>1 → 2</em>
<em>2 → 3</em>
<em>3 → 4</em>
<em>4 → 5</em>
In this function, every input has one output and each x value is different.
