In Becquerel’s initial experiment, in which he exposed uranium salt crystals to sunlight, the photographic paper became exposed even though it was shaded from the sun by a dark cloth. At first, how did Becquerel explain this result?

A) The black cloth emitted gamma rays that exposed the film.
B) The sun emitted gamma rays that exposed the film.
C) The crystals emitted X-rays when they were exposed to light, exposing the film.
D) The crystals phosphoresced when they were exposed to light, exposing the film.