Red blood cells are created deep inside our bones in the . 4. Your are not visible because they are under your skin - b

Bone Marrow

Explanation:

Red blood cells are created deep inside our bones in the bone marrow

Bone marrow is the soft, spongy tissue inside the bones that makes blood-forming cells (blood stem cells)These cells turn into blood cells including: White blood cells to fight infections,red blood cells to carry oxygen throughout the body,platelets to control bleeding

Hair root is not visible because they are under the skin

Hair is a keratinous filament growing out of the epidermis which is primarily made of dead, keratinized cellsStrands of hair originate in an epidermal penetration of the dermis called the hair follicleThe part of hair which is anchored in the follicle, lies below the surface of the skin and is referred to as the hair rootThe hair root ends deep in the dermis at the hair bulb,and includes a layer of mitotically active basal cells called the hair matrix

Mg is used to measure mass