How does Einstein's explanation account for each of these characteristics of the photoelectric effect?

a. The photoelectric current is zero for frequencies below some threshold.

b. The photoelectric current increases with increasing light intensity.

c. The photoelectric current is independent of \(\Delta V\) for \(\Delta V \geqslant 1 \mathrm{~V}\).

d. The photoelectric current decreases slowly as \(\Delta V\) becomes more negative.

e. The stopping potential is independent of the light intensity. Which of these cannot be explained by classical physics? Explain.