An object is placed 20cm in front of a thin lens. A real, inverted image forms on a screen 40cm on the opposite side of the lens. Which of the following best describes the lens?
a) A converging lens, because it produced a real inverted image on the side opposite the object.
b) A converging lens with focal length +40cm, obtained by substituting a negative image distance into the thin-lens equation.
c) A diverging lens, because the focal length calculated from the lens equation must be negative for a diverging lens.
d) A converging lens, because it produced a reduced real image on the side opposite the object.