Which option is the sealed joint for air service in underground pipes?

• A. A joint designed to leak for testing.
• B. A joint for attaching fixtures above ground.
• C. A joint that resists water pressure.
• D. Sealed joint for air service in underground pipes.

Answer: (D)

In underground sprinkler piping, there is a dedicated air service connection that lets you introduce air to fill and test the line before water is used. The joint used for this purpose must be sealed so it can hold the air pressure during the test and prevent air from leaking out or water from entering the line afterward. That’s why the sealed air service joint is the correct choice—it directly describes the access point created for testing and keeps that access securely closed. The other ideas don’t fit: a joint designed to leak would undermine the test; a joint for attaching fixtures above ground isn’t related to underground air testing; and while joints in underground lines must withstand pressure, this option isn’t about the specific air-service access used for testing.