Final answer:
Victaulic plain and couplings should not be used with plain end or welded pipe and fittings.
Step-by-step explanation:
The correct answer is C) Plain; welded.
Victaulic plain and couplings should not be used with plain end or welded pipe and fittings. These couplings are designed for use with grooved end pipe and fittings to provide a secure and leak-proof connection.
For example, if a student wants to join two pieces of grooved end pipe together, they can use a Victaulic coupling to make the connection. However, if the student tries to use the same coupling with a plain end pipe, the connection will not be secure and may lead to leakage.