Final answer:
Adhesive fittings and compression fittings are not suitable for gas piping systems due to their inability to provide a secure and leak-proof connection for the high pressures present in gas lines.
Step-by-step explanation:
The fittings that cannot be used on a gas piping system are adhesive fittings and compression fittings. Threaded and welded fittings are commonly used in gas piping systems because they provide a secure and leak-proof connection that is suitable for the high-pressure conditions that can be present. Adhesive fittings are not suitable for gas lines as the adhesive can degrade over time, leading to leaks. Compression fittings are also not recommended for gas lines because they do not provide a seal that is as secure as that provided by threaded or welded fittings.