Which of the following is NOT necessary for testing a double check valve?

In the context of testing a double check valve, the primary components that need to be addressed are the test cocks that are specifically designed for evaluating the functionality of the valve. A double check valve typically has two test cocks, which are used to measure the pressure on either side of the valve in order to determine if it is functioning correctly and providing adequate backflow prevention.

Opening the first and second test cocks allows for the necessary measurements to be taken. These test cocks facilitate the assessment of the pressure differential across the valve, which is critical to ensuring that the valve is sealing properly and is capable of preventing backflow. Each test cock serves a distinct purpose in the testing procedure to ensure compliance with industry standards and operational efficiency.

In contrast, the existence of a third or fourth test cock would generally not pertain to standard double check valve testing. As such, these additional test cocks are not typically present in a standard configuration for double check valves, making it unnecessary to open them for the required testing process. Therefore, the focus is solely on the relevant test cocks for assessing the performance of the double check valve, confirming that option D is not necessary for the testing procedure.