Pipe nipples are _____ on both ends.

The correct answer is that pipe nipples are threaded on both ends. Pipe nipples are short lengths of pipe that are used to connect two fittings. The threads allow them to be easily screwed into other components such as valves or other segments of piping. This threading provides a secure and leak-proof connection, which is essential in refrigeration systems and other plumbing applications.

Threaded connections are advantageous because they can be disassembled easily for maintenance or replacement without the need for specialized tools or equipment. The use of threading also allows for versatility in connecting various components of different sizes and types. This is particularly important in refrigeration systems where space and configuration can vary widely.

In contrast, flanged ends would require matching flanges and bolts for assembly, which is not characteristic of a pipe nipple. Closed ends would imply that the pipe nipple is capped off, not usable for connection, while welded ends suggest a permanent bond that cannot be easily disassembled. Therefore, threaded ends are the defining feature of pipe nipples, making them suitable for a wide range of applications.