How does the carbon dioxide necessary for photosynthesis enter plants?

Carbon dioxide, present in the air, enters the leaves by "diffusion" through the stomata - pores. The amount of carbon dioxide available can fall when planting is dense, when plants have been rapidly photosynthesising, or in unventilated greenhouses, so growers can artificially supply it by burning propane gas or releasing pure carbon dioxide, in greenhouses or poly tunnels.

Eventually carbon dioxide is again released as a by product of photosynthesis, thus completing the circle.

Plants pull in CO2 though their leaves - it diffuses though small holes in the underside of the leaf - these are called stomata. This is also where the oxygen leaves the plant.

The correct answer is b