Where does gas exchange occur in a plant?

Gas exchange in a plant occurs through the tiny pores called stomata found in the leaves of plants.

Gas exchange occurs during photosynthesis. Carbon dioxide is taken from the atmosphere through the stomata and oxygen is released as a waste product.  Gas exchange is essential for the  plant's survival.

Normally stomata open when the light strikes the leaf in the morning and close during the night. Opening and closing of stomata is guided by two guard cells which are found on each side of the stomata.  When the guard cells become turgid,  they bend into a crescent shape allowing the stomata to open. When they lose turgor pressure, they become flaccid and return to their original shape and the stomata close.

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.