A small lightbulb is slowly moved along the principal axis toward a concave mirror, starting from a very large object distance. At which region does the image change from real to virtual?
a) When the bulb is closer than f to the mirror
b) Precisely when the bulb is at distance 2f from the mirror
c) Precisely when the bulb is at distance f from the mirror
d) When the bulb is between 2f and f from the mirror