The $C_4$ and $C_3$ plants differ from each other in

Which of the following statements is correct regarding $C_4$ plants?

Blackman demonstrated that increasing illumination increased the photosynthetic rate up to a point when $CO_2$ becomes limiting. If light was not limiting,temperature becomes limiting. Emerson found that maximum $CO_2$ fixation could be achieved with brief flashes of light. Mark the correct statement in the following:

Which of the following is a unique event that provides the basis for life on this planet?

Statement-$I$: When $PS-II$ is functional,the electron is circulated within the photosystem and photophosphorylation occurs due to the cyclic flow of electrons.
Statement-$II$: The stroma lamellae membranes lack $PS-I$ as well as $NADP$ reductase enzyme.

Which of the following is correct for photosynthesis?