The wave picture of light has failed to explain which of the following phenomena?

The figure shows the variation of photocurrent with anode potential for a photo-sensitive surface for three different radiations. Let ${I_a}, {I_b}$ and ${I_c}$ be the intensities and ${f_a}, {f_b}$ and ${f_c}$ be the frequencies for the curves $a, b$ and $c$ respectively.

Consider the two following statements $A$ and $B$, and identify the correct choice given in the answers :
$(A)$ In photovoltaic cells, the photoelectric current produced is not proportional to the intensity of incident light.
$(B)$ In gas-filled photoemissive cells, the velocity of photoelectrons depends on the wavelength of the incident radiation.

Light with a frequency of $1.5$ times the threshold frequency is incident on a photosensitive material. If the frequency is halved and the intensity is doubled,what will be the photoelectric current?

Define saturation current,stopping potential,and cut-off frequency in the context of the photoelectric effect.

$A$ photoelectric cell is illuminated by a point source of light $1 \ m$ away. When the source is shifted to $2 \ m$,then: