Why Are Epithelial Membranes Not Very Thick

Why are epithelial membranes not very thick?

A) Epithelial tissues are required to support other cells growing on top of the tissue.
B) Epithelial tissues play a critical role in transport,and must be thin to allow diffusion to occur.
C) Epithelial tissues can only reach a certain thickness because their growth is constrained by microtubules inside the cell membrane.
D) Thick epithelial tissues would collapse due to the drying effect of the environment.
E) Epithelial tissues are programmed to stop dividing after only a few cell cycles.