Which graph shows a function whose inverse is also a function?

Using the concept of inverse function, it is found that the first graph shows a function whose inverse is also a function.

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A function f(x) will only have an inverse function if: , that is, a value of y will have only one respective value of x.

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A similar problem is given at link

A good way to determine if a line represents a valid function is to use the vertical line test.

To do this, you imagine a vertical (up and down) line moving across your graph from left to right. It should only be touching the line at one point at a time.

If it is touching more than one point on the line at a time, the line is not a valid function.

The first line and its inverse both pass the test.

The second line passes the test, but its inverse does not.

The third line also passes the test, but again, its inverse does not.

The same applies to the fourth line and its inverse.

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what is the image?