The outer layers of human skin are composed of dead cells impregnated with keratin and oil, which make the epidermis relatively impermeable to water, yet humans sweat freely in hot temperatures. this occurs because:
(a) the salt in sweat allows it to diffuse through the skin.
(b) sweat glands have special channels through the skin.
(c) an osmotic gradient in sweat moves it through the skin.
(d) sweating occurs in only those areas of the body where the skin is water permeable.

B. Sweat glands have special channels through skin

Explanation - The upper layer of skin is made up of epidermis and dead cells which is impermeable to water. However, we are able to sweat during hot weather because the sweat glands have specific opening on the surface of the skin called as pores.

The sweat is discharged through these pores. Sweat also has dissolved salts in it but it does not allows it to diffuse through the skin rather it helps in maintaining the osmoregulation of the body.