What is used to soften the joining surfaces when solvent cementing plastic pipe?

In the context of solvent cementing plastic pipe, primer is used ahead of the cement application to clean and soften the joining surfaces of the pipe and fitting. This chemical preparation is crucial as it prepares the surfaces for a better bond. The primer effectively softens the plastic, allowing the solvent cement to penetrate more effectively. The cement itself, upon application, further fuses the softened surfaces of the pipe and fitting together, creating a strong and seamless joint.

While both primer and cement play vital roles in the joining process, it's the combination of both that ensures a successful connection. The primer softens and cleans, while the cement actually joins those softened surfaces. Therefore, the correct answer recognizes the importance of using both primer and cement in achieving a reliable bond in plastic piping systems.