Answer the following question given this paragraph: Metal is regularly used in the human body for hip and knee implants. Most implants need to be replaced over time because, among other things, metal does not bond with bone. Researchers are trying to find better metal coatings that would allow metalto-bone bonding. One challenge is to find a coating that has an expansion coefficient similar to that of metal. If the expansion coefficients are too different, the thermal stresses during the manufacturing process lead to cracks at the coating-metal interface. Another example of thermal stress is found in the mouth. Dental fillings can expand differently from tooth enamel. It can give pain when eating ice cream or having a hot drink. Cracks might occur in the filling. Metal fillings (gold, silver, etc. ) are being replaced by composite fillings (porcelain), which have smaller coefficients of expansion, and are closer to those of teeth. Q: What material is regularly used in human hip and knee implants? A:

OPTIONS:
i. wood;
ii. glass;
iii. coal;
iv. metal;
Answer:
iv.