Answer with explanation:
Usually while dealing with two dimensional shapes, we use line as a symmetry.Consider two dimensional geometrical shape
1. Square
it has four lines of symmetry,
two lines from middle ,one perpendicular to Base and other perpendicular to Side which is equal to Base.
2. Line segment
You can find line of symmetry of line segment, by putting a point in between, but in actual line is a 3 dimensional geometrical shape, so line is better way to find line of symmetry of line segment.
→→And when, we consider three dimensional shapes, we use plane instead of lines to find symmetry of three dimensional shapes.
For Example, Cylinder
If we cut a Cylinder from between , that is by plane which is horizontal to cylinder,we get two equal section of cylinder.
Number of symmetry
1. Line
2. Plane