At present, the raw materials used in artificial joints are nothing more than metals and plastics. Because of potassium, sodium, chlorine and other chemical substances in the human body, metal materials can corrode and rust, and plastics age. Therefore, the chemicals of metal and plastic must be highly selected. stable. What does this passage mainly support? ['Artificial joints must be made of metal.', 'Plastic artificial joints are prone to aging.', 'Potassium, sodium, chlorine and other chemical substances in the human body are active and have a corrosive effect.', 'Artificial joints must be made of highly stable metals and plastics.']