At present, the materials used for artificial joints are nothing more than metals and plastics. Because potassium, sodium, chlorine and other chemical substances in the human body can make metal materials corrode and rust, and plastics age, the chemical properties of the selected metals and plastics must be highly stable. What is the main point of this passage? ['Artificial joints must be made of metal.', 'Plastic artificial joints are prone to aging.', 'Human potassium, sodium, chlorine and other chemical properties are very active and have a corrosive effect.', 'Artificial joints must be made of highly stable metals and plastics.']