Final answer:
Schedule 40 PVC and CPVC as well as PVCSDR pipes should not be threaded due to insufficient wall thickness which compromises structural integrity when threaded.
Step-by-step explanation:
The correct answer to the student's question is option C, Schedule 40. Schedule 40 PVC and CPVC as well as PVCSDR should not be threaded because their walls are not thick enough to provide sufficient strength for the threads. Threading these materials may compromise their structural integrity and lead to leaks or breaks under pressure. Instead, these types of pipes are generally joined using solvent cement or by the use of slip-on or compression fittings. It is important to follow the manufacturer's recommendations and local building codes when working with these materials to ensure proper installation and safety.