The most vital process for the existence of life on Earth is

$I.$ 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?

Select the incorrect statement $:$

Which of the following does not evolve oxygen during photosynthesis?

During daylight hours,the rate of photosynthesis is higher than that of respiration,and the ratio of oxygen produced to that of consumed is (in $:1$)

Select the correct option:
$(1)$ $C_4$ plants have double the efficiency of carbon fixation compared to $C_3$ plants.
$(2)$ Sugar is transported from source to sink through the $\text{phloem}$ / $\text{xylem}$ tissue.