On double cylinder assemblies, one must ensure?

The correct choice highlights the importance of securely attaching the cylinder tray to the unit's frame on double cylinder assemblies. This attachment is crucial for safety and stability purposes. A properly secured tray helps avoid mishaps such as the cylinders tipping or falling during use, which can lead to serious accidents or injuries.

In scenarios where multiple cylinders are used, ensuring that the tray is firmly attached enhances overall system integrity and functionality. The tray serves as a support structure that holds the cylinders in place, preventing movement that could compromise the system.

While considerations like the installation of the joiner pipe, equal sizing of cylinders, and the state of the valves are certainly important for operational efficiency, the structural stability provided by the attachment of the cylinder tray to the frame is foundational. Without this fundamental step, the risk associated with the use of double cylinder assemblies increases significantly.