What is the common value of $PQ$ (photosynthetic quotient) of a leaf?

The graph given below shows the effect of potassium cyanide on the rate of photosynthesis of Chlorella at different light intensities. Which one of the following can be deduced from the graph?

The rate of photosynthesis does not depend on .........

Photosynthesis is maximum during:

Photosynthetic rate and productivity will increase by increasing $CO_2$ concentration in :-

Identify the incorrect sentences:
$(I)$ Humidity in the atmosphere is relatively higher in areas where light becomes a limiting factor for plants.
$(II)$ An increase in light intensity beyond a certain point causes the breakdown of chlorophyll,leading to a decrease in the rate of photosynthesis.
$(III)$ $C_4$ plants respond to higher temperatures and show higher rates of photosynthesis,while $C_3$ plants have a much lower temperature optimum.