A student is unable to project the image of a candle on a screen using a thin concave lens, no matter how she adjusts the setup. Which statement best explains this observation?
a) A concave lens always forms real images behind the lens.
b) Because the focal length of a concave lens is positive, the image distance becomes negative and a screen cannot be placed there.
c) Rays refracted by a concave lens diverge, so no real image can be formed on a screen.
d) Light from a candle is incoherent and therefore cannot be focused by a concave lens.