Question

A triangular pyramid is cut by a plane. In which case is the cross section a quadrilateral?


when the plane is perpendicular to the base and passes through the apex



when the plane is parallel to the base of the pyramid



when the plane intersects the base and two lateral faces of the pyramid



when the plane intersects the base and all three lateral faces of the pyramid

Answer 1

When a plane intersects a plane, it forms a line.
A triangular pyramid has four faces.
To form four straight lines and therefore a quadrilateral, the intersecting plane must intersect all four faces.
intersects 3 faces, forming a triangle.