x __ Coal and oil are __ A skateboard and roller blades are substitutes; substitutes complements complements substitutes complements substitutes; substitues

Coal and oil are complements. A skateboard and roller blades are substitutes.<br /><br />Explanation:<br />- Complements are goods that are used together, such as coal and oil, where coal is used to generate energy to refine oil.<br />- Substitutes are goods that can replace each other, such as a skateboard and roller blades, where one can be used instead of the other for the same purpose.