Regulating gas exchange

We call convex lens pores formed between guard cells stomata.

Guard cells are thinner on the ventral side and thicker on the dorsal side. When absorbing water, the guard cells expand and the two cells separate, while when the guard cells lose water, the stomata open and close. In gas metabolism such as respiration and transpiration, stomata become channels for air and water vapor, and their throughput is regulated by the opening and closing of guard cells, which has important physiological significance. Known as the "gateway" of plant transpiration and the "window" of gas exchange.

So choose: D.