I would say that the most likely answer is C.
While B and D could happen that are too vague and non-descript, A is completely wrong.
Pretty sure the answer to this question is neurons or nerve cells.
Erosion shapes. Lowest level directs.
You may not have cystic fiborosis to still be a carrier of it. You would not know if you were a carrier until you have a child that is born with it.
Without concentration you can't have diffusion, because diffusion is basically the spreading out of molecules which are concentrated, or compacted.
