A rectangular pyramid is sliced so the cross section is perpendicular to its base and passes through its vertex. What is the shape of the cross section?

The answer for the question shown above is: The shape of the cross section is a triangle.

The explanation is shown below:

Imagine that you have a plane that intersect the rectangular pyramid and it passes through its vertex and it is perpendicular to the base of the pyramid, when you look the cut, you will see a triangle as a cross section.