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You have external defense systems, which include your skin, tears, mucus, saliva, stomach acid, cilia (small hairs) and helpful bacteria. The skin, nails and hair form the integumentary system, which stops dirt and pathogens from entering the body.
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A. containers A and B represent mixtures
<span> All the millions of different proteins in living things are formed by the bonding of only </span>20 amino acids<span> to make up long polymer chains.</span>
A gateway substance is something that may be menial but could lead to something bigger, an example is weed in the drugs world, weed is not hard to get and is usually a gateway drug to stronger types of drugs and potentially more dangerous