According to an estimate,how much carbon is fixed annually in the biosphere through photosynthesis?

Diffusion, a process that occurs along the concentration gradient, is actively involved in:

Plants in which the first product of $CO_{2}$ fixation is $C_{3}$ acid,i.e.,the ...$A$... pathway,and those in which the first product was $C_{4}$ acid $(OAA)$,i.e.,the ...$B$... pathway. Complete the given statement by filling appropriate options in the given blanks.

Is it correct to say that photosynthesis occurs only in the leaves of a plant? Besides leaves,what are the other parts that may be capable of carrying out photosynthesis? Justify.

$A -$ Productivity of $C_3$ plants is higher than $C_4$ plants.
$R -$ Photorespiration occurs in $C_3$ plants.

$I.$ In $C_{3}$-plants,the Calvin pathway takes place in mesophyll cells.
$II.$ In $C_{4}$-plants,the Calvin pathway takes place in the mesophyll cells.
Which of the following statements is true?