$\"\"$

Horizontal-line Test : If every horizontal line intersects the graph of a function $\"\"$ in at most one point, then $\"\"$ is one-to-one function.

Redraw the diagram by taking horizontal line intersect the graph.

$\"\"$

Observe the above diagram.

Since horizontal line intersects the graph of a function $\"\"$ in every point, the function $\"\"$ is not one-to-one function. $\"\"$

The function $\"\"$ is not one-to-one function.