Final answer:
Piercing-type valves are suitable only for copper tubing because copper's softness allows for easy piercing without risking damage like cracks or leaks, unlike PVC, steel, or aluminum tubing.
Step-by-step explanation:
Piercing-type valves should only be used on copper tubing. This is because copper is a soft metal that can easily be pierced without causing cracks or splits, which could lead to leaks. Piercing-type valves are not ideal for use on PVC, steel, or aluminum tubing due to their material properties. PVC can crack, while steel and aluminum are generally too hard for the piercing tool to penetrate effectively without causing damage to the pipe.