A Convex Spherical Surface with Radius R Separates a Medium

Multiple Choice

A convex spherical surface with radius r separates a medium with index of refraction 2 from air. As an object is moved toward the surface from far away along the central axis, its image:

A) changes from virtual to real when it is r/2 from the surface
B) changes from virtual to real when it is r from the surface
C) changes from real to virtual when it is r/2 from the surface
D) changes from real to virtual when it is r from the surface
E) remains real