To form an ellipse, what is the relationship between the base of the cone and the angle of the plane that intersects the cone?

a
The plane is parallel to the base of the cone.

b
The plane may be perpendicular to the base of the cone.

c
The plane is at an angle that is neither parallel nor perpendicular to the base of the cone.

d
The plane must be at a 45° angle to the base of the cone.