The 4th one is the answer
Each cell becomes a thin plate; these plates pile into layers to form the nail. As with hair, nails form by keratinization. When the nail cells accumulate, the nail pushes forward. The skin below the nail is the matrix.
Hair is made of a tough protein called keratin. A hair follicle anchors each hair into the skin. In the hair bulb, living cells divide and grow to build the hair shaft. Blood vessels nourish the cells in the hair bulb, and deliver hormones that modify hair growth and structure at different times of life
Nails start in the nail root, hidden under the cuticle. When cells at the root of the nail grow, the new nail cells push out the old nail cells. These old cells flatten and harden, thanks to keratin, a protein made by these cells. The newly formed nail then slides along the nail bed, the flat surface under your nails.
There must be
45 electrons, because the number of protons equals the number of electrons
