While mosses can cover a significant area of land, they are never more than a few centimeters tall. why do they not grow tall?

They have no vascular system.

Explanation:

Vascular system contains an important feature they have series of tubes which helps to transport nutrients, and water over a distance in plants. The vascular system of phloem, and xylem allows trees (such as redwood) to grow a hundred feet tall.

Mosses do not have vascular system. Without the vascular system liverworts, and mosses are not able to grow large. Mosses are rarely one inch in height. Mosses are small flowerless waxy plants with no stem, and leaves.

the answer is a or when it says crossing over

i think its the first one