Which material is not permitted for use in underground plumbing systems?

The correct answer pertains to the distinction between general PVC and specific types of PVC that are rated for underground use. PVC, which stands for polyvinyl chloride, is a widely used plumbing material due to its durability and resistance to corrosion. However, when it comes to underground applications, not all PVC types are suitable. Only those that are specifically rated for underground use can be utilized because they are designed to withstand the unique pressures and conditions of being buried, including soil movement and moisture.

Materials such as ABS (acrylonitrile butadiene styrene), cast iron, and copper have been established as acceptable for underground plumbing systems under the proper guidelines. ABS is known for its lightweight and excellent performance in drainage systems; cast iron has longstanding use due to its strength and sound-dampening properties; copper is reliable and resistant to corrosion.

Thus, the emphasis on PVC in the context of underground installations highlights the necessity for compliance with specific standards to ensure functionality and safety. Therefore, only PVC that meets the criteria for underground use is permissible, making it clear why the option that indicates PVC without that rating is the one that is not allowed.