The Main Problem Physicists Had with Understanding the Photoelectric Effect

Multiple Choice

The main problem physicists had with understanding the photoelectric effect before Einstein explained it in terms of photons was:

A) the intensity of emitted electrons did not depend on the intensity of the source.
B) the maximum energy of the emitted electrons did not depend on the frequency of the source.
C) the maximum energy of the emitted electrons did not depend on the intensity of the source.
D) the cutoff frequency depended on the material used as a target.
E) the cutoff frequency did not depend on the material used as a target.